You gain a gelatin-like alternative form whose description is left up to you. Congrats kid, you're a slime now!
*As a slime, your body is almost impervious to physical blows, strikes, and slashes. You can still be harmed by energy attacks, magic, and basically by just about any really volatile substance.
*As a slime, you no longer age, making you immortal.
*Your consciousness is now spread throughout your body, so you no longer require your brain to think. Don't worry, you won't lose any IQ points if bits of you were to get splattered over the terrain or whatever.
*You gain a ridiculous ability at shapeshifting. Squish yourself flat to get under a door, turn a finger into a key, shapeshift into someone, etc. You're even able to shift the pigmentation of parts of your body to whatever color you want.
*You can invade and control the bodies of others, puppetting them around and talking through them. This is, for obvious reasons, less effective against robots, other electronic enemies, and some types of undead.
*Alternatively, you could form a sort of shell or suit around the victim, and control them that way. This approach is also more effective against the beings described above.
*You can shift your mass through your hosts body, enlarging certain features and/or parts. Somehow, this never negatively effects the host's health.
*You can turn others into slime people too! They gain all the abilities of this essence, and are utterly loyal to you, seeing you as a sort of "queen slime".
*You can consume objects, like rocks or leaves, to increase your mass. For some reason, this seems to only work on inanimate, non-sentient material.