A Mantle is a unique title that can be given to an individual, granting powers and personality changes. Examples are Mother Winter, Winter Queen, Winter Lady, and the Winter Knight with corresponding versions for the Summer Court.
These can actually be built in GURPS using the Spirit Vessel rules from GURPS Thaumatology, or the Totem-Bearer rules from GURPS Powers: Totem and Nature Spirits.
Some tweaking of the power modifiers may be needed but this ability is well represented by the Alternate Form advantage.
The Winter Knight mantle for example can be removed for ignoring or violating "Winters Law" and grants several traits and personality changes. Since the personality changes over time I would probably use the Spiritual Distortion rules from GURPS Thaumatology or the Corruption rules from GURPS Horror which are essentially a refined variant of the distortion rules from Thaumatology.
As a mantle is designed to be worn most, if not all the time it fits Totem-Bearer better than the more exhausting Totem advantage from GURPS Horror: The Madness Dossier or GURPS Monster Hunters: Holy Warriors which add an ongoing FP cost. However the Requires Skill Roll is inappropriate for Dresden Mantles (though might work for other concepts), and Shamanic, Mediated works but might be renamed.
- Shamanic, Mediated, -5%
- Corrupting, -20%
- Reduced Time 5. +100%
- Total +75%
Thus a Mantle costs 27 points plus it's pool cost.
The granted abilities of the new form are not entirely known but for the Winter Knight probably include High Pain Threshold, Heroic Reserves, a magical ER limited to winter powers and perhaps other toughness related traits. Note that it does not need to include a healing ability as he seems to heal normally (though Wizards heal a little faster in the setting) and as an Alternate Form it swaps out the paralysis he has in his normal form.