Hollow Marauder’s first ability doesn’t change its mana value, which is always 7.
2024-04-12
When Hollow Marauder’s triggered ability resolves, the next target opponent in turn order (or, if it’s a target opponent’s turn, that opponent) chooses a card in hand without revealing it, then each other target opponent in turn order (if any) does the same. All chosen cards are then discarded at the same time.
This spell costs less to cast for each creature card in your graveyard.
Flying
When this creature enters, any number of target opponents each discard a card. For each of those opponents who didn't discard a card with mana value 4 or greater, draw a card.
less to cast for each creature card in your graveyard.