Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-24108046-20131214122819/@comment-10759846-20131218203826

Before he died he infected Lydai with a portion of his spirit which allowed him to give her instructions on how to fully revive his corpse.

How she did was by knocking out Derek with Wolfsbane Powder on a Worm Moon dragging him the Hale House and relfecting the Moon light on Peter who then bites Dereks Arm.

Better Summary here:

http://teenwolf.wikia.com/wiki/Party_Guessed