During your Main Phase: You can Fusion Summon 1 Fiend-Type Fusion Monster from your Extra Deck, using monsters from your hand or field as Fusion Materials. You can only use this effect of "Dark Contract with the Swamp King" once per turn. If Summoning a "D/D" Fusion Monster this way, you can also banish monsters from your Graveyard as Fusion Material. Once per turn, during your Standby Phase: Take 1000 damage.