I know that it's an old mission but I picked it up out of my abandoned missions. The Dark Below I'm on the quest step for the Will of Crota and after completing the strike it says it is incomplete. I know you have to change it to 320 light difficulty and I have, competed the strike about 4 times now.
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I'm surprised, normally people make the exact opposite mistake as you did. The "will of crota" step of The Dark Below questline requires the completion of the level 16 strike, not the 320 variant. The quest you're thinking of is Variations on a Theme.