"Abyss Ragnarok" can be sent to the Graveyard via cards such as "Dark Contract with the Swamp King" and "Dark Contract with the Witch", then Normal Summon "D/D Nighthowl" and use its effect to Special Summon "Abyss Ragnarok". Prior to Xyz Summoning this card, "Nighthowl" can be Tributed to use the effect of "Abyss Ragnarok" to banish a problematic monster your opponent controls.
This card can be Xyz Summoned by first using the effect of "D/D Swirl Slime" to use itself and a Level 8 "D/D/D" monster in your hand as Fusion Material to Fusion Summon a "D/D/D" Fusion Monster, then banishing "Swirl Slime" to Special Summon "Abyss Ragnarok" from your hand, and using its effect to Special Summon the Level 8 "D/D/D" monster from your Graveyard that was sent there with the effect of "Swirl Slime".
This card can easily be Xyz Summoned if there is a "D/D Savant Thomas" in your hand, "D/D Swirl Slime" in your Graveyard, and a "D/D" card in your Pendulum Zone. Use the effect of "Swirl Slime" to banish itself and Special Summon "Thomas". Then, use the effect of "Thomas" to destroy that "D/D" card in your Pendulum Zone and Special Summon a Level 8 "D/D/D" monster from your Deck. Overlay "Thomas" and the monster that was Special Summoned with its effect to Xyz Summon this card.
This card can more easily be Xyz Summoned by using the effect of "D/D Berfomet" on another "D/D" monster you control, changing its Level to 8. Then, another Level 8 "D/D" monster can simply be Summoned to fulfill this card's requirements.
Despite requiring specific Materials, "Xyz Universe" can also be used to Special Summon this card, making it usable in a great variety of Decks.