Polymorphy

Miserable piles of lies, these bodies made of flesh.

They could become anything. Then, they chose magnificent wings to soar through infinite skies, strong claws to keep their burrows safe, keen eyes to better watch the stars. But when it came time to rest, they had forgotten who they were, before.

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Polymorphs can alter the Aer that makes up their own body to change its shape and turn into their wildest imagination. There is effectively no limit to the forms a Polymorph can take. However, the bigger the changes, the more Aer they require to be maintained over a given period of time. The shapeshifting act is one of balance, as one's body is used to a certain form and starts to leak Aer as soon as this form is broken. Polymorphs must then train to perfectly control their transformations as to avoid any such leak : the more precise the transformation, the least likely it is to leak at all.  
Also, while it is easy to entirely transform into a chair or into a dog, these complete shifts are extremely dangerous : depending on they ways the Polymorph uses to channel their Aer, they may not be able to change back given their new body. Moreover, turning into an inanimite objects usually results in the polymorph dying instantly, as such objects lack the organs necessary to sustain life. 
The last drawback of Polymorphy, and probably its biggest, is that the Polymorph keeps the exact same Arcane Signature when transformed. Meaning that can be still recognized even if they copy the appearance of another person. Anyone familiar with the Polymorph or that other person can then instinctively tell that they are not who they claim to be.