This is how I ran these games: http://adeptplay.com/actual-play/some-osr-sandbox-play (in English)
I am going to tell about two linked one-shots; the first was open table, essentially gaming on demand, at a con and I am describing the last of the sessions, while the second session was an online oneshot.
The system in use is as described above. The rules framework is described here, and quite sketchy: https://ropeblogi.wordpress.com/2020/04/15/vanhan-liiton-henkiset-saannot/ (in Finnish). Those are the player-facing rules. On the referee side there are random encounters, reaction rolls, restocking dungeons, and so on, all this in accordance with the methodology described behind the first link.
I ran games on demand on three days of Ropecon. I had chosen an essentially first level dungeon, as the idea is to run with an open table and not spend much time on set-up; just pick or bring a character and play for as long as you care to and then leave. I had a stack of pre-generated characters, since I have run games like this for some time and decided finally that it does not make sense to have people create new characters, since I can recycle the old ones, too. There were one Pathfinder, a couple of Dungeon world and some D&D 5 characters there, too, in addition to the ones from varying homebrew rule sets I have used over the years. They all have attributes on the D&D range, some notion of level and some notion of hit points. The attributes varied from character to character.
The ground level of the dungeon was "The forbidden fortress", for the Dungeon squad! rpg, though it had already been cleared more than once, abandoned again and restocked, this all maybe twice. The adventure does not contain a dungeon below the ground level, but does have some creatures that might be living there; these I added as random encounters to the next level, which was and is a starter dungeon from some quick start rules for Sword and wizardry. The levels below that would use Jeff Rients' Under Xylarthen's tower, but none have ventured so deep.
I did the restocking after the expeditions on Friday and Saturday. A creature from the Dungeon squad! adventure, fungi folk, is not otherwise very dangerous, but it does have a particular special ability: "Once per combat, Fungi-Folk may forgo an attack in order to secrete a blast of reproductive spores. Doing so creates another d6 Fungi-Folk within 1 round." Exponentially increasing amounts of mobile fungi are interesting enough an idea, of which I had to make some sense. I decided that sending forth the instantaneously reproductive spores is a stress response which the fungi use in face of persistent threat, and then they stampede the threat and the newly sprouted fungi feed on them to not wither and die in near future. Still not exactly hard science, but at least there is a logic to how the creatures work.
Another thing to appear during the re-stocking was a reference to the general encounter tables I have, from which the source to use this was "The game of dungeon" (a very old variant of D&D), and of the creatures therein a dragon was selected. This one had lots of hit dice and striking it with various weapons was more or less difficult; hitting it with a spear was very gard, but dealt ten dice of damage, whereas a usual attack deals one die of damage. I converted the difficulty numbers for hitting it into defense values in my game. The dragon came with a treasure worthy of a dragon, totalling at several tens of thousands of gold worth. And since it was a big beast, it had obviously broken all the doors on its way and widened out some doorways and left some tracks, as one would expect.
This was all set-up. Game starts, new players come, characters are given out, they go into the dungeon. They use hand-outs (maps of the previous group) to decide where to go. A corridor with a bunch of giant mushrooms, on one of which there hangs a sack that makes clinking noises. A poke with spear at one of the shrooms, it takes a step back (pretty good reaction roll, so the shrooms are not aggressive). After some precautions one of the characters passes the shrooms carefully, is not molested, so others follow. The presumed treasure is left there. They are about to walk past a room, but there is a huge serpentine dragon there! It just watches them but lets them pass (another good reaction roll). Some more careful looks inside reveal that the creature is coiled on quite a lot of coins and other treasure. What could first level characters decide but to slay the beast?
People take places on both sides of the doorway, strong folk with big weapons closest. One provokes the dragon (maybe shoots at it, maybe by speech; the details escape me) and it lunges forward! As it comes on, it is struck by the people in ambush. The pretty tight doorway limits the dragon's movements severely, as does pinning it in place by stabbing it with a great sword. A few characters are wounded, one severely: once you run out of hit points, the remaining damage goes to your relevant attribute and you can choose to have the character die, sparing them the suffering, or make an attribute roll to avoid lethal damage. This one failed and also failed the roll to stay conscious. Luckily a spearman finally hits the dragon, and in a very critical manner, and slays it. First aid rescues the character with broken ribs, as their lungs are not penetrated, but they are still incapable of action. They do wake up at some point after lots of trouble getting them out of their maille.
The death throes of the dragon are formidable and since the doorway was quite tight, it breaks that. I roll for how much damage the structure itself takes (an ad hoc d6 roll with possible results declared before rolling and then a roll in open) and it does not quite come down, but is close. The ceiling from close to the doorway crumbles down and there is now unstable roof and an unstable heap of rocks separating the characters from the treasure. Luckily no random encounters appear. I am pretty explicit about the risks but they choose to have one climb over and then start moving the treasure. It takes quite some time, but aside from some hit point damage due to falling rocks, everything goes smoothly. They get a magical sword (it glows) and sacks of treasure. The way back should not be difficult. There is an improvised solution for helping the wounded character and those who can are carrying sacks of treasure.
The mushrooms are still there and have done exactly nothing while all the dragon-slaying was going on. The players discuss the matter and end up suspecting that they are just biding their time and will strike soon enough while the vulnerable characters are among them. This is of course wrong; the previous reaction check still stands. The players don't however have no way of knowing that the mushrooms are essentially peaceful rather than treacherous and it seems that roleplayers have a deep-seated suspicioun towards mobile shrooms, so there we are. Treasure is discarded, ranks formed, shrooms attacked. The shrooms are quite impotent in combat, unless there is sufficiently many that they can form a troop and crush you against something, but the dice are against the players. Then the shrooms spout spores to make things even worse.
The game gets quite chaotic. Another player arrives, this time with a pre-generated catwoman for D&D 5, so I decide they have come in and discover the combat. One character is among the shrooms, doing their best to stay on top of them and not be battered between or crushed under them. Furthermore, we finally get a random encounter: a fleurian pact warlock (homebrew warlock variant with powers related to mushrooms and plants for D&D 4 from "Open game table", a collection of rpg blog entries in book form from the time people wrote blogs). I roll for their level (default adventurer level: half a chance of level one and if not half a chance of level two and if not half a chance of level three etc.) and stats; they are level one with poor stats, but they are still a caster with the three spells casters start with by these rules, which I roll from the first level powers in the article, and they are dangerous. For further chaos, the game is run in a noisy con environment so I have to explain everything twice to be heard and I am also running the game bilingually, one of the players has some problems with spatial thinking or is otherwise a bit slow. Things happen, but it is not smooth.
To cut things short, eventually one character manages to climb over the charging shrooms, the shroom witch attacks from rear and is not quite killed, the player of the character with broken ribs finally lets their character die in peace after being smashed against the wall by the shrooms. The cat-woman fires a couple of arrows, sneaks in to see the witch kill the last of the other characters, and makes a final bid to assassine it. Awful luck with dice rolls, the shrooms discover them and death follows. I rolled reaction of the witch towards both the characters and the shrooms, assigning significant modifiers due to the mushroom affinity, and it went the predictable way.
Eventually one character barely got away with a magical sword and all the others were killed. I updated the and restocked the adventure as usual, but there was no more time to run the game in the con.
Some time later my typical game did not happen, so I decided to run a follow up. Here is the information that was available before the session: https://ropeblogi.wordpress.com/2020/06/15/the-dragons-hoard-and-the-terrible-mushrooms/ (in English). The three players I know from before are not new to this way of playing and are quite skilled. The fourth one was a stranger and quite passive during play; I don't think they really clicked with what we were doing. I played with them in some Old school essentials play later, and they were quite passive there, too, so maybe they just are more of a passive player. I am not comfortable with further psychoanalysis of the person from afar.
We used Discord for voice and text, Google sheets for characters and an online whiteboard for drawings.
During the restocking most of the shroomfolk and the witch had been assigned to the dragon's chamber, which necessiated that more of the roof had collaped and the place had found a stable configuration. Some other mushrooms and the sack of coins they had been carrying were in a dark room in the interior of the former tower. The players had a bunch of maps and notes from previous players as hand-outs (see the linked blog post); these represent what they heard from the traumatized survivor.
Into the dungeon, some orientation, avoid antagonizing some dog-sized lizards that happen to come in (a random encounter), find a room with big mushrooms and go in carefully, digging around a bit, until a sack with coins is found. A hundred or two of gold, if I remember correctly, so a formidable treasure already for new characters. They move forward, find the treasure and more of the shrooms (which have not moved yet). One goes in, discovers the witch, is surrounded by the shrooms. The reaction roll is not bad but the witch is careful. The character asks if they can take the treasure; answer is no without some compensation; something to feed the mushrooms with, which the players immediately interpret as farmer corpses (the immediate assumption of ill will from walking mushrooms and their allies is quite remarkable; they would have been fine with typical kitchen waste and less valuable pieces of animal carcasses or whatever, but nobody thought of asking). The holes in the roof/ceiling are also noticed.
Player characters go back to town and use the treasure (and some charisma rolls) to recruit folk and equip them with spears, and also buy lots of coal, oil and torches. Half climb to the roof, half go indoors and make a fiery barrier with spears behind it, while the others wait for smoke and then start throwing down the fuel and the torches. The mushrooms send froth their spores due to the severe attack. I dice for how great a proportion of the new shrooms end up forming up there, lifted by the heat, and it is a significant number. The rest are in chaos first and then the witch manages to organize them to charge/march towards exit; they do suffer losses from the fire and the spears, but charging through such obstacles is their (only) strength, so it goes well enough. The indoors group has to retreat to the next doorway, where they make quite a formidable stand.
Up on the roof there is chaos as suddenly mushrooms appear. I ask for instinct rolls (one the attributes) for situational awareness, some player character figures out what is happening, but their charisma rolls for leadership are not sufficient to have the untrained villagers instantly respond. A general roll for how well it is going up there ends up strongly against the humans, so the shrooms manage to form a troop and crush and batter some people, including, as it happens, one player character who dies falling down. Another attacks them but fails very critically, stumbling and falling down into the blaze with some parts of the roof. They make their rolls against severe wounds and unconsciousness, so there they are, battered and with a broken wrist, and manage to get out of the hot and smoky area while still conscious.
The mushrooms up there are eventually defeated (they suffered losses, then they no longer had enough for a formation and after that they are quite trivial opponents). The witch has escaped outside in the chaos (I rolled for this on the side). Inside there is a killing ground where mushrooms are slashed into pieces as they try systematically pushing in through a doorway blocked by their disassembled compatriots. The fire starts to threaten, so there is an escape. The witch has killed some stragglers outside and tries striking at those who running from the fire, but manages to only daze one before having to run into the woods. A few mushrooms also manage to escape the fire, tumbling off the roof (I roll for their fate, too, because exponentially growing mushrooms are quite a key element for further play, if left unchecked).
The first floor of the dungeon is now a smoldering heap of stone and rocks, with entrance further down blocked. Peasants are hired to excavate and separate into pieces the partly melted gold, which is all the remains of the treasure; still some twenty thousand worth after paying the workers. To determine the value, I erased everything that does not survive heat, reduced the value of sceptres and crowns and whatever to a small fraction, and then deducted maybe one fourth as labour costs. Most of the rest was divided among the player characters and hirelings who were in the thick of it. It was still more than 2000 gold coins per player character and half that for each surviving hireling, which is quite a lot.
The first interesting matter is the loose rules system used. The player-facing rules consist of attribute rolls (exploding d20+attribute against difficulty level), hit points or sisu (d6, reroll with an increasing number of penalty dice when taking a break), damage (d6 from most attacks with actual wounds after hit points run out and then risk of death and unconsciousness), spells and character advancement. The difficulties of rolls are set on ad hoc basis, but they are typically cleared with the group and this is done before dice are rolled. Players can choose to take back actions after we discuss the difficulty (or dispute the rulings on grounds of realism) and often I ask their opinion about matters like how much do they think it would cost to get all the fuel they want.
The framework is, in particular, loose enough that having a Pathfinder or D&D 5 character along is not a problem, not to even mention characters from older editions. In case of the cat-woman I used D&D 5 rules to adjudicate that character's actions, since I had the mental capacity for it in that case. Otherwise I would have defaulted to the same framework I used for the others.
The second interesting matter is how we saw risk and reward work, as well as player skill in terms of decisions made. I thought the dragon would be deadly, but no, it turns out that attacking it while its mobility was limited was bold and, with some good rolls, successful. I could do nothing but see the hit points on the creature go steadily down and then be finished with a spear to the brain. The players could have walked past it, or could have charged it where it could fight effectively, or done any number of other things. They could have had bad luck. Some battle diagrams can be seen in the linked blog post as handouts for the online game.
There were bold decisions with respect to getting the treasure, and then the decision to attack the mushrooms, which though somewhat explainable, turned out to be bad. If they were not magical multiplying mushrooms and if the witch had not appeared, they might have prevailed, at least with some luck, so the defeated was caused by understanding the situation in a wrong way (wrong in the sense of not corresponding to the fictional reality; with the information they had, it was understandable).
The other group was careful, cashed in, and leveraged the fortune to spectacular success, but also destroyed much of the treasure in the process. There could have been options for peaceful interaction, but this time we did not see them.