Source: Eberron - Rising from the Last War

Wondrous item, very rare (requires attunement)

This opalescent, symbiotic goo comes sealed in a jar and slowly shifts and moves, as if endlessly exploring the jar's interior. To attune to this item, you must first drink the contents of the jar, unlocking the following properties.

Resistant. While attuned to Kyrzin's ooze, you have resistance to poison and acid damage, and you're immune to the poisoned condition.

Amorphous. As an action, you can speak a command word and cause your body to assume the amorphous qualities of an ooze. For the next minute, you (along with any equipment you're wearing or carrying) can move through a space as narrow as 1 inch wide without squeezing. Once you use this property, it can't be used again until the next dawn.

Acid Breath. As an action, you can exhale acid in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 36 (8d8) acid damage on a failed save, or half as much damage on a successful one. Once you use this property, it can't be used again until the next dawn.

Symbiotic Nature. The ooze can't be removed from you while you're attuned to it, and you can't voluntarily end your attunement to it. If you're targeted by a spell that ends a curse, your attunement to the ooze ends, as it seeps out of you.

If you die while the ooze is inside you, it bursts out and engulfs you, turning your corpse into a black pudding allied with the daelkyr (see the Monster Manual for the black pudding's stat block).