The Summon Deercoy spell creates a deer at the point of casting. The deer will walk back and forth on any solid surface and attract enemy fire. Upon taking enough damage, it will explode.
It is unlocked once the Orb under the Snow Chasm has been collected.
The Summon Deercoy spell was originally named Summon Deer Decoy, but was renamed after a petition on reddit caught the developers' attention.