Noble Phantasm: Norse Wind: He Sees You When You're Sleeping (Rank A, Anti-Person)
Sooner or later, children learn the truth about Santa Claus: he doesn't exists, the presents they receive come from their parents.
This is not the truth.
With the crystallization of his legend, Saint Nick is capable of possessing any sleeping person in the world (it is said that in his natural form he's capable of possessing a great number of people at once, but the Servant container does impose certain limitations), guiding their actions in a somnambulistic trance. People under such influence experience what happens to them as hazy dreams, which, once they wake, are recontextualized into memories that make sense to them.
As such, Saint Nick is capable of guiding parents to buy just the right presents for their kids and make them think it was their own idea accomplished under their own will... or use this ability for more sinister purposes.
Servants are immune to this Phantasm, and powerful magi could, in principle, resist... if they somehow realize what's happening to them in the first place.