If you followed his explanation, he "shifted his cloak" (or more simply "shapeshifted") to wyvern once he was in contact with one to copy its form.
If you followed his explanation, he "shifted his cloak" (or more simply "shapeshifted") to wyvern once he was in contact with one to copy its form.
I'm going to go a step further and say he "merged" with it some how.