The soul is made of water.

[Lunar Origins]

Deep Sea Soul Keeper, Tyranus
WATER / Level 10
[ Fairy / Effect ]
Cannot be Special Summoned, except by its own effect or by the effect of "Merging Soul Ritual". If a WATER monster on the field is targeted (except for an attack): You can Special Summon this card (from your hand), then send all other monsters on the field to the Graveyard. During the turn this card is Summoned, you cannot attack. You can only control this face-up monster. You can discard 1 card and send this card you control to the Graveyard; Special Summon 1 monster from either player's Graveyard. If this card would be destroyed, you may activate this effect at that time.
[ ATK: 3000 / DEF: 2600 ]

Deep Sea Servant
WATER / Level 4
[ Aqua / Effect ]
When this card is Summoned: Target 1 card you control; send that card to the Graveyard, then add 1 "Soul Keeper Initiate, Tyra" from your Deck to your hand.
[ ATK: 1800 / DEF: 600 ]

Soul Keeper, Tyra
WATER / Level 6
[ Aqua / Effect ]
Can be Normal Summoned without Tributes. If this card is sent from the hand to the Graveyard: Add 1 WATER monster from your Deck to your hand.
[ ATK: 1200 / DEF: 2000 ]

Merging Soul Ritual
[ Spell / Quickplay ]
Banish 2 Aqua-type WATER monsters from your hand, field, or Graveyard: Special Summon 1 WATER monster from your Deck whose Level is equal to the total Level of the banished monsters.

[Bahamut - Envoy of the End]

Boss Monster: Targeted for an effect, attack or both? Feel the attack is too high for something that nuke everything so easily (in a way that gets round Stardust et all). At least stop it attacking if its summoned that way for a turn. Revival effect would be brilliant if it was a quick effect.

Spell Card: Hmm... sees potential for that as splashable (shot) in many water decks.

The cards have some synergy, but maybe not as much as intended I think.

[Lunar Origins]

Well, edited effect, nerfed it and then boosted it.