Retributive Psychoplasm

HD 12
AC as unarmored
Attacks * (attacks as HD 6)
Speed as human
Regenerates 1 hp per round

The retributive psychoplasm is just a huge, noxious mass of repugnant jelly-flesh, limned with rugose folds and rings of cillia.  It spends its time sleeping at the intersections of dungeons, where it's 10' diameter bulk forces adventurers to step over it if they want to bypass it without disturbing it.  It is a sleepy blob and will not wake up until it takes damage.  It will attack whoever damaged it, unless that is impossible/unknown, in which case it will attack whoever is nearest until the area is quiet once more.

If the PCs attack it, it gains an attack of the exact same type.  (So attacking it with a 1d8 greataxe will give it a 1d8 slicing pseudopod attack.)  It can gain up to 6 attacks in this way, including ranged attacks like arrows and slingstones.  It can even gain elemental attacks, spitting out a 15' cone that does 3d6 damage (save for half) of the appropriate type, once it takes fire/ice/acid/lightning damage.

It is immune to the last three types of damage that it has taken.

Dungeon mooks (especially yoblins) are fond of them because they are ugly, smelly, and vaguely threatening (traits they appreciate) and they can safely ignore the beasts (even walking on them) while adventurers often attack them.

How to Use: Put it in an intersection.  See if your players attack it.

Alternatively, put it in an environment where they can get enemies to attack it, so that it will go kill their enemies for them.

Alternatively, put it in an environment where they risk pissing it off.  Spikes falling from the ceiling.  A situation where the wizard really wants to use a fireball on the goblins standing atop the psychoplasm.