Pyeryem is the magic of Ussura, gifted to the chosen by Grandmother Winter
The Source of Pyeryem is Grandmother Winter, the - supposed - daughter of Theus. She gives the magic and she can take it away or, more likely, curse you with it.
Pyeryem is activly used and accepted in Ussura, but disliked in the rest of Western Theah.
Pyeryem is the ability to transform into another creature. To do this, you must obtain "permission" from an individual of the species you wish to emulate. when you transform, you change into an exact duplicate of the individual you obtained permission from.
Stage 1: You have the power to speak with animals of any sort. Creatures from Ussura tend to be more intelligent then creatures from outside it's borders, but any animal can "speak" with you. You can request their aid and permission to "wear their skin", but they are not required listen to you or do as you ask.
Stage 2: You may now transform into any creature you received permission from. These include mundane creatures (such as wolves and bears) to more exotic and magical creatures (such as Sidhe and Drachen) Getting the permission from each of these creatures - especially the more powerful ones - is incredibly difficult. In this form, your animal instincts take some control, leading you to do things you would not do as a human. This process is exhausting, as is the process of returning to human form.
Stage 3: You many now take only aspects of specific creatures you have obtained the skins of, such as the wings of a bird, the claws of a bear, or the gills of a fish) and incorporate them into your human form. This is less exhausting then a full transformation, but the gained abilities are les effective then the full form (e.g. you fly slower, have less force behind the claws, and cannot swim any better)