Unlike most abilities that use devotion to determine the magnitude of an effect, Mogis's Marauder uses devotion to determine the number of targets its ability has. Use your devotion to black as you put the ability on the stack. Mogis's Marauder's mana cost will always count toward your devotion to black. Once targets are chosen, it doesn't matter if your devotion to black changes while the ability is on the stack. The number of targets is locked in.