Kapitel 1651: New Predator

The [Formless] Ability allowed Serpent to assume the form of any shadow resting in Sunny's soul, inheriting its powers and abilities. The shadow had to be of the same or lower Rank and Class, which meant that the most powerful form Serpent could currently assume was that of the Sybil of the Fallen Grace… the Terror of LO49. 

And, in fact, that form was suited for Sunny's goal much better than the Mountain King. However, Serpent had to maintain the transformation with its own essence, and could not sustain such a powerful shape for a long time, let alone constantly.

Once the Shadow's essence was exhausted, the effects of those powers it had assumed would be gone. In other words, if Sunny had ordered Serpent to turn into the harrowing Sybil and enthrall a swarm of Nightmare Creatures, those abominations would have been released from the mind hex before too long.