After some more thought, here is my shot at Erlking's magic.
Drawing on the power of the Erlking, you are able to cast spells that fit his essential nature: wildness, the hunt, balefire, bloodlust, death. This magic is under the sway and watch of the Erlking; making use of it will inevitably catch his notice.
Cost: 4 refresh for the package, not to mention the approval of the Erlking. Reduce this cost by 1 if you already have Evocation or Thaumaturgy; reduce the cost by 2 if you have both.
Benefits: Standard sponsored magic benefits.
In Addition, the Erlking's magic may be used as an element for evocation, allowing evocation effects that encourage wildness, the hunt, balefire, bloodlust, and death. This includes the ability to produce effects along the lines of divination tracking spells but with an evocation spells methods and speed. The Erlking's evocations always include the otherworldliness of the hunt in some way. Vines to trap will have thorns, balefire is eerie and green, a tracking spell may scare near by animals.
I'm not sure of a Catch for the Erlking, nor if there is anything he'd effect more than normal. I'm tempted to go with Winter, since the use of fire and the end of hunting in many places once winter starts make me associate the Erlking a little more with Summer. But this seems weak.