2 "D/D" monsters If another "D/D" monster(s) is Special Summoned to your field, while you control this face-up card (except during the Damage Step): You can target 1 "D/D" monster in your Graveyard; Special Summon it. You can only use this effect of "D/D/D Flame King Genghis" once per turn. If this card is destroyed by battle, or if this card in its owner's possession is destroyed by an opponent's card effect: You can target 1 "Dark Contract" card in your Graveyard; add it to your hand.
You can Fusion Summon D/D/D Flame King Genghis with D/D Swirl Slime’s effect. Banish Swirl Slime from your graveyard to Special Summon a D/D monster in your hand and trigger Flame King Genghis’ effect filling your field with monsters.
When Fusion Summoning a Flame King Genghis with something like Dark Contract with the Swamp King, you can use D/D Vice Typhon as fusion material. You can use Vice Typhon’s effect to immediately Fusion Summon another D/D/D monster and trigger Flame King Genghis’ effect. This combo requires a lot of setup.