Theusaes, the Atlantean Revolutionary
Level 5 Water Sea Serpent/Effect
1750 Attack, 1600 Defence
Once per turn, you may banish all WATER monsters of level 3 or less in your discard pile, and send all of your banished WATER monsters to the discard pile; This card gains 200 Attack for every card currently banished by this effect, and 200 Defence for every card currently in the discard pile by this effect.
Alright. First things first, the discard pile is not a term in Yu-gi-oh, so change all instances of that phrase to "Graveyard." Additionally, replace all instances of "May" with "Can". The swapping of banish and grave is kinda cool, but it can be pretty easily broken in the right hands, as it has the potential to trigger massive amounts of effects. Overall, its a good boss monster, with potentially worrying applications.
Atlantean Commonfolk (x3)
Level 2 Water Sea Serpent/Effect
400 Attack, 600 Defence
When this card is sent to the discard pile, Special Summon a WATER Monster of Level 3 or below to the field, and add 1 WATER Monster of level 4 or less to your hand.
So... I have 2 questions, and no matter what the answers are, this card will probably be very broken. Anytime this card is sent to the grave (From anywhere, including banish pile, meaning it triggers off Theusaes) you get two things. For free. But, I dont know where you get these things from. Do you get them from your deck? Do you look into your trade binder and pick the rarest one? Do you drive to Konami HQ and protest outside the building until they print you the exact card you want? I need to know the locations of this cards targets, but no matter what they are, it doesn't look super balanced.
Deep-Sea Geyser (x3)
Continuous Magic Card
On your turn, you may activate one of the following effects-
-When one of your banished monsters would be sent to the graveyard, you may instead send it to your hand.
-Draw a card; If it is WATER Monster of level 3 or lower, discard it. If it is a non-WATER Monster card, discard it, and discard this card.
I'd suggest making this guy a trap as opposed to a spell, but other than that, It ought to be fine once its PSCT is better.
Atlantean Elite Guard (x2)
Level 4 Water Sea Serpent/Effect
800 Attack, 1800 Defense (Are only the base 2 custom cards limited to 1/2 of the Signature's attack?)
When played, If "Theusaes, the Atlantean Revolutionary" is not on your side of the field or in your hand, tribute this card: Special Summon "Theusaes, the Atlantean Revolutionary" from your deck or graveyard to your side of the field. If this card is in the Graveyard and "Theusaes, the Atlantean Revolutionary" is targeted by an attack or effect, you may banish this card; negate the attack or effect.
Alright, this guy probably shouldnt be able to be recycled as much by your boss monster. Shuffle it into the deck instead of sending it to grave, perhaps. Otherwise your monster is going to be very incredibly tanky.