Abyss is the final boss of Marvel Vs. Capcom 2. He/it is said to be the cause of an evil wind that blows across the land killing all the plants in its path, which is what triggers the playable characters in the game to investigate this phenomenon. Abyss is "entropy incarnate" and is a force capable of killing off all life and returning the world to primordial times.
Abyss' true form is a floating glowing sphere, but it can turn into three other forms.
The first form is a giant suit of armor with glowing green markings.
The second form is an amorphous humanoid figure made out of slime.
The third and final form is a gigantic three–headed dripping red demon beast with green slime oozing out of it's mouths.
After all three forms are defeated, Abyss' sphere breaks and the player character and Ruby Heart drop it to the bottom of the ocean.