Old School informed GURPS Dungeon Fantasy gaming. Basically killing owlbears and taking their stuff, but with 3d6.
That looks like WOTC 5e trade dress. A quick Google says it's from a Kobold Press product, so there must be a license.
While a photo of this creature from Kobold Press’s Creature Codex is on this tweet, I’m willing to bet this image came from Tumblr.
Now make that castle a mini dungeon in its own right and you've got some serious fun.Castle in the OozeLocation: Wandering MonsterEntrances: Get EatenRandom Encounters: 1-6 on 1d: Shark!Or maybe you get access to the mini dungeon once you kill the ooze and defeat the shark. The idea of a wandering dungeon is kind of a fun thing to pull from hex crawls.
Yes, yes, yes. This.Although by "mini dungeon" you mean "full sized castle that can only be entered by those shrunk to miniature size," right? So it's really a full castle-dungeon setting but inside a shark-filled ooze? Of course you do. What else could you mean?