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I think it means that while he remains in play then your opponent has to discard down to seven at the end of each turn rather than 8, or at least that's always how I've played it (and when I say "played it" I mean played it years ago so I might be remembering wrong). Although, I'm kinda wondering what would happen if he was removed from the game or captured... As long as he's not negated or interrupted beforehand I feel like his effect would still remain.