This skill lets you shapechange into a Spawn, and if so desired, maintain yourself in that state or resume your normal shape before you normally would. This is the hallmark skill of the Disciple guild, but only a fully fledged member of one of the chaos guilds can truly master it.
Using this skill to become a spawn in the first place may fail and is dependent on your skill level. Spawning and despawning are more stressful than simply maintaining spawn form. Using this skill to maintain spawn form always works.
The ingame help reads:
Skill duration: 6. Type of skill : neutral skill. Affecting stats: None. It uses 75 endurance points. True followers of chaos have been known to assume the form of an unambiguous monster. This may only be legend - however, a terrible legend about monsters with too many limbs... You can also use 'despawn' as target. Using the skill while already spawned will replenish your spawn. Despawning and respawning causes additional strain on your endurance.