Tried to make this one OP, tell me what you think.
Rank 7, Dark, Voidling / Effect Type, ATK 3000, DEF 2600
3 Level 7 Voidling-Type and/or "FTK Phantom" Monsters
Once per turn, (Quick Effect): When a Spell/Trap or Monster Effect is activated: You can negate that effect, and if you do, attach that card to this card as Xyz Material. You can detach 2 Xyz Material from this card; Banish all other monsters with ATK less than or equal to this monster's ATK. You cannot Special Summon the turn you activate this effect. When this card leaves the Field; Special Summon 1
Level 7 Voidling-Type or "FTK Phantom" Monster from your GY.
Much too overpowered. This is similar to that one card demonstrated in Custom Cards as to how not to design. You could drop down the Materials to 2 Level 7 and bring down the effect. My suggestion:
2 Level 7 Voidling or "FTK Phantom" Monsters
Once per turn, (Quick Effect): When a Spell/Trap is activated: You can detach 1 Xyz Material from this card; negate that effect, and if you do, destroy it. If this card leaves the field: Special Summon 1
Level 7 Voidling or "FTK Phantom" Monster from your GY.
We've already a Main Deck monster that can negate all three. But at least that one has a cost to negate. This one was much too reminiscent of that Red Princess card.
So I gave this one another shot, I did change the focus of the effect as FTK Phantom - Vavel Elreigner already negates effects, thus the new focus of this card is to special summon. I want to play with the idea of this Archetype having multiple play styles so to do so I wont be making Ritual Support, and have my cards focus on "FTK P V E" and Xyz Summons, hope you guys don't mind.
Banish 1 "FTK Phnatom" monster you control. Special Summon 1 "FTK Phantom - Vavel Elreigner" from your Deck, or GY. If you control "FTK Phantom - Vavel Elreigner", except the turn this card was sent to the GY: You can banish this card and 1 "FTK Phantom" monster from your GY, then Special Summon 1 Level 7 or lower "FTK" monster from your Deck. You can only use each effect of “Soul of Darkness Unleashed” once per turn. You cannot Special Summon monsters from the Extra Deck, except "FTK Phantom" monsters, the turn you activate either of this card's effects.
I've a suggestion. Make the second effect say "except FTK Phantom - Vavel Elreigner".