Ouroboros

It has its own tail in its mouth and it has never let go. The circle it makes is large enough to enclose a village and old enough that there are trees growing inside it that nobody planted. It is not guarding anything and is not waiting for anything. It is eating itself and growing at exactly the rate it is being eaten, and inside the ring that principle applies to everything else as well, which is the difficulty.

Coil Inward: bludgeoning, melee 30 ft, +7 to hit, 3d10+6, and a second sweep of the body, 3d8+5. It does this without releasing the tail and without appearing to move. Close the Ring: 60 ft, save Dex Quick, fail save roll 3d8+5 bludgeoning as the circle tightens, cooldown 2 rounds. Creatures that fail are inside it.

What Is Taken Returns: at the end of each round everything inside the ring returns to the state it was in at the start of that round. The ouroboros returns to its HP. So do the party - wounds close, spent spells return, broken things are whole. Death Is Not Exempt: a creature reduced to 0 HP or killed inside the ring is alive and whole again at the end of the round. Nothing that happens in there stays happened. Leaving Locks It In: a creature that crosses out of the ring before the round ends keeps everything from that round permanently - its wounds, its spent resources, and its death if it took one. Carrying a dying friend out is therefore exactly the wrong instinct, and the DN should let the party work that out rather than telling them. Reduced to 0 HP within a single round, which requires the whole party to land everything at once, it lets go of the tail, and what happens then is the DN's to decide and should be enormous.