Pokemon Prism: Because we Love Rainbows

[PTGigi]

That sounds like a good C:

You could potentially have differing sets needed for different Legendaries either different amounts or different types. Or maybe even just different requirements like Mewtwo needs you to have X number of "Purple" (range specified) Pokemon, Raikou needs X number of "Yellow" Electric Pokemon, Ho-oh needs you to have 3 complete rainbow sets, Lugia needs 2 complete sets of a Water/Psychic/Flying type, etc.