What's the name of the trope that means fae can't cross over unless a human crosses over first?

Question

There's a trope I remember, having a denizen of the fae realm only be able to cross over to the human world once a human has first crossed over to their world. Though I remember seeing it multiple times, I can only remember one specific instance of it: in Terry Pratchett's Lords and Ladies, the unicorn only crosses over when a hunter wanders in.

(I think it may also be in the Dresden files? I can't recall)

Answer 1

I recall this as one of the (many) clauses of The Compact -- a very old agreement between the Seely and Unseely courts that protects the world of men from the Faerie. The Compact is seen in many works that deal with the Fae, from Peter Beagle to Lost Girl, but I can't tell you where I encountered this clause.