Ochre jelly

An ochre jelly is an amorphous creature that resembles a dark yellow amoeba.

Special abilities
An ochre jelly has 100% immunity to electrical and physical damage, as well as immunity to critical hits, fear, knockdown, level drain, ability drain, mind-affecting spells, paralysis, poison, and sneak attacks. It has permanent true seeing.

This creature's attack has a 5% chance of triggering an on-hit effect that will hold its target for 5 rounds if a DC 14 saving throw is failed.

When an ochre jelly is physically attacked or targeted by certain spells (specifically hammer of the gods, storm of vengeance, or the electrical spells ball lightning, chain lightning, electric jolt, Gedlee's electric loop, great thunderclap, or lightning bolt), it splits into two creatures. These creatures are small-sized, have fewer hit dice, lack the on-hit hold ability, and have their damage immunity dropped to 75%. In addition, each creature is damaged so that it has only half the hit points of the original creature at the time of the split. Under the normal or higher difficulty settings, the new creatures will again split under these same conditions, this time into tiny creatures with damage immunities dropped to 50%.