It is immune to:
- acid (half-celestial)
- cold (half-celestial)
- electricity (half-celestial)
- fire (half-dragon)
It has sonic resistance 20 (axiomatic) and Spell resistance 25 (axiomatic).
It is also immune to mind-influencing effects, poison, sleep, paralysis, stunning, and polymorping [Note: Some of these might disappear when it gets Intelligent, it is AFAIK not covered by the rules]
It has wings and can fly, and breathe a 6D10 (DC20) cone of fire (1/day). It has bite and 2 claws, each doing 2D6 (2D4 claw) + 9 + 2D6 acid damage. And it will destroy weapons and armor with acid. It's also reasonably intelligent (INT 5, WIS 5).
But the real killer is the black puddings immunity to weapons, if it is damaged by any weapon it will instead split into two identical creatures, each with half the original's hit points, but with all special abilities.
It's going to be a bitch to damage, only a few spells work on it (e.g. force and death), and those must beat SR. And it doesn't matter how much melee damage you do, its only going to split it into two monsters.