You can Ritual Summon this card with “Rebirth of Nephthys”. You can only use each of the following effects of “Devotee of Nephthys” once per turn ● If this card is Ritual Summoned: You can Special Summon 1 “Nephthys” monster from your Deck, also destroy this card during the End Phase. ● If this card is in your GY: You can destroy 1 “Nephthys” card in your hand, and if you do, Special Summon this card.
Rebirth of Nephthys is this monster’s intended Ritual card and if you use another Devotee of Nephthys or Cerulean Sacred Phoenix of Nephthys as ritual tribute you can destroy one card on the field.
The Djinn monsters are all able to use themselves as ritual tribute even while in the graveyard, which eases your summoning cost. Additionally, each of them gives your Ritual monster an additional effect when they are used as ritual tribute.
Here are the monsters you can special summon from your deck with Devotee of Nephthys’ first effect. You can also destroy these cards to revive Devotee of Nephthys with her second effect while also triggering the destroyed monster’s effect. Destroy Sacred Phoenix of Nephthys so it can revive itself on your next turn.
Well, Cerulean Phoenix is a 3000 ATK monster that goes to 3300 ATK with Beatdown and can pop any monsters your opponent controls: I wouldn't say Maximus is the biggest issue here. Koa'ki Meiru Wall and Ice are way bigger threats honestly since they can be Special Summoned from the Deck by Urnight consistently.
I guess Disciple of Nephthys is a fine addition to Yubel, but I'm mainly seeing a Pure Nephthys and a Geartown Nephthys Deck.
You probably don't want to run the Ritual Monsters in a Yubel Deck, but I might be wrong: I'll look into every option.