Amorphous form

Amorphous form was a transmutation spell that changed the target into a oozy mass, like a pudding or puddle, letting them slip through gaps with ease.

Users
Circa 1372 DR, an arcane version of amorphous form was known to be used by Underdark assassins, sorcerers and wizards, and prime Underdark guides. By 1374 DR, a divine version was available to clerics of Ghaunadaur, Lord of Slime, who'd been initiated into the most dreadful secrets of the cult.

Effects
On casting amorphous form, a target being touched by the caster became a puddle-like mass with a fluidity that resembled an ooze. The target's belongings were also affected. The target lost none of his or her agility and could still move. The new fluid body allowed them to enter narrow spaces as small as 2 inches (5 centimeters) in width. They also gained the ability to move freely and without having to breathe in water.

While in this form, the target was completely immune to poison, form-changing magic, and stunning effects. The target lacked a discernible anatomy and they were impossible to effectively flank.

The spell had a number of downsides. While under the effect of amorphous form, the target could not attack or cast spells that required any somatic, material, or verbal components or foci.

Components
Amorphous form required somatic and material components. If cast as a spell of divine magic, it required a divine focus. The material component was a pinch of gelatin. On casting the spell, the gelatin was sprayed on the target.

Items
A staff of the Underdark allowed the casting of amorphous form among others.