They started out small fish, but have quickly become a larger school in the ocean that is Yugioh. With the release of Structure Deck: Realm of the Sea Emperor, Atlanteans have quickly flooded the OCG with their unique effects, swarming, and synchro/Xyz spamming.
Here are the cards of the archetype:
Level 2 WATER
The original Atlantean, from The Shining Darkness.
Level 3 Water
When this card inflicts battle damage to your opponent: You can Special Summon 1 level 4 or lower Sea Serpent-type "Atlantean" monster, except "Atlantean Marksman", from your deck. When this card is sent to the Graveyard for the activation of a WATER monster's effect: Target 1 set card your opponent controls; destroy that target.
Atlantean Heavy Infantry
Level 2 WATER
While this card is face up on the field, once per turn, during your Main Phase, you can Normal Summon 1 level 4 or lower Sea Serpent-type monster in addition to your normal summon or set. (You only gain this effect once per turn). When this card is sent to the Graveyard for the activation of a WATER monster's effect: Target one face-up card your opponent controls, destroy that target.
Atlantean Attack Squad
Level 3 WATER
While you control another face-up Fish, Sea-Serpent, or Aqua-type monster, this card gains 800 ATK.
Level 4 WATER
While this card is face-up on the field, level 3 or lower Sea-Serpent-type monsters can attack your opponent directly. When this card is sent to the graveyard for the activation of a WATER monster's effect: Add 1 Sea-Serpent-type monster from your deck to your hand, except "Atlantean Dragon Riders".
And the big boss monster:
Atlantean Dragon Poseidra
Level 7 WATER
During your Main Phase: You can tribute 3 level 3 or lower WATER monsters you control; Special Summon this card from your hand or Graveyard. When this card is Special Summoned by this effect: Return all Spell/Trap cards on the field to the owner's hand, and if you returned 3 or more cards, all face-up monsters your opponent currently controls lose 300 ATK for each card returned.
Atlanteans like Atlantean Marksman, Atlantean Armed Soldier, and Atlantean Dragon Riders, can utilize their search and destroy effect when sent to the graveyard for the activation of a WATER monster's effect, and this makes the basis for the deck, quickly filling the hand and destoying cards. The question arose of whether or not the Atlanteans would get their effects, as their effect language clearly states "send" rather than "discard" found on many of the WATER monster effects that fit into the deck. To clear this up, all that matters is the fact that Atlanteans are sent to the graveyard to activate an effect, so discarding for cost effectively sends the Atlanteans to the graveyard to activate an effect. Atlanteans also get their effects when they are detached as Xyz material, or sent from the deck to the graveyard, either through milling or through effects, like Genex Undine's effect.
WATER Monsters whose discarding effects will activate Atlantean's effects:
Abyss Soldier- Discard a WATER monster to bounce a card from the field to the owner's hand.
Abyss Warrior (OCG- Level 4 Effect Monster, WATER/Aqua, 1800/1300)- Discard a WATER monster to place a card on the field and place it on the top or bottom of the deck.
Skreech- When destroyed by battle, select and send 2 WATER monsters from the deck to the graveyard.
Genex Undine- When this card is Normal Summoned, send 1 WATER monster from the deck to the graveyard to add one "Genex Controller" from the deck to your hand.
Brionac, Dragon of the Ice Barrier- Discard any number of cards to target the same number of cards on the field, Return them to the owner's hand(s). EDIT: As of September 1, 2012, Brionac is Forbidden in Advanced Format play, but can be used in Traditional Format.
Mermail - Abysspike (OCG, from Abyss Rising- Level 4 Effect Monster, WATER/Fish, 1600/800)- When this card is Normal or Special Summoned: You can discard one WATER monster to the graveyard; add 1 level 3 WATER monster from your deck to your hand. The effect of "Mermail - Abysspike" can only be activated once per turn.
Mermail - Megaloabyss (OCG, from Abyss Rising- Level 7 Effect Monster, WATER/Sea Serpent, 2400/1900)- During your Main Phase: You can discard 2 WATER monsters, except this card, Special Summon this card from your hand. When this card is Special Summoned by this effect: Add 1 "Abyss" Spell or Trap card from your deck to your hand. You can tribute 1 other face-up attack position WATER monster you control; this turn, this card can attack twice during the Battle Phase.
Mermail-Abysslinde- (Level 3 WATER/Aqua Effect Monster, 1500 ATK/1200 DEF): When this card on the field is destroyed and sent to the graveyard: You can Special Summon one "Mermail" monster from your deck, except "Mermail Abysslinde". You can only use the effect of "Mermail Abysslinde" once per turn.
This card searches and special summons "Mermail Abysspike" to gain field and card advantage, searching out "Abyss-Sphere" from the deck.
Moulinglacia the Elemental Lord- (Level 8 WATER/Sea Serpent Effect Monster, 2800 ATK/2200 DEF): Cannot be Normal Summoned/Set. Must be Special Summoned (from your hand) by having exactly 5 WATER monsters in your Graveyard, and cannot be Special Summoned by other ways. When this card is Special Summoned: Discard 2 random cards from your opponent's hand (or the entire hand if less than 2). You can only use the effect of "Moulanglacia the Elemental Lord" once per turn. If this card leaves the field, skip the Battle Phase of your next turn.
This card can be searched by Atlantean Dragoons, and is an instant drop formidable boss monster that can surprise your opponent. The graveyard can easily be filled with effects from Genex Undine, Mermail-Megaloabyss, and Mermail-Abysspike.
Other cards that have a great impact on this deck:
Deep Sea Diva: When this card is Normal Summoned, you can Special Summon 1 level 3 or lower Sea-Serpent-type monster from your deck.
Deep Sea Diva is a driving engine for this deck. It can search out an Altantean, Special Summon it, plus it's almost like a free Synchro/Xyz summon. Most Atlanteans are level 3 or lower Sea-Serpents, so this card is a no-brainer in an Atlantean build.
Tour Guide from the Underworld: When this card is Normal Summoned: You can Special Summon 1 level 3 Fiend-type monster from your hand or deck. It's effects are negated, and it cannot be used as a Synchro Material monster.
Newly Semi-Limited, but can still be used with Sangan for an easy Xyz monster (or Sangan can be used to search more Atlanteans), or with Cloudian - Poison Cloud in keeping with the WATER theme of the deck. Tour Guide is splashable in most decks, so it seems that it would fit here.
Dragon Ice: When your opponent Special Summons a monster, you can discard 1 monster from your hand to Special Summon this card from your hand or Graveyard. There can only be 1 face-up "Dragon Ice" monster on the field.
You can discard Atlantean monsters for Dragon Ice's effect, or you can discard Dragon Ice itself to Special Summon this card (like Machina Fortress). It's Special Summon still allows you your Normal Summon for other monsters like Deep Sea Diva or Tour Guide. Dragon Ice combos with Atlantean Dragon Riders to search out other Atlanteans, or with Armsman/Armed Soldier to stop opponent's plays.
Genex Controller: The other part of the Genex engine, Controller is retrieved when using Undine's effect. Even though it is a Normal Monster, it is a tuner, and can be used for Synchro Summoning.
Genex Ally Birdman: You can Return one face-up monster you control to the hand to Special Summon this card from your hand, and this card gains 500 ATK if it is a WIND monster that is returned. If this card is Special Summoned with this effect, banish it when it is removed from the field.
Genex Ally Birdman serves 2 purposes in this deck: to bounce Atlanteans/Mermail monsters back to the hand to potentially reuse their effects, and to aid in Synchro Summoning.
Also, there is a 2 card combo with Genex Ally Birdman/Deep Sea Diva (provided by Dudewitbow):
Deep Sea Diva> Atlantean Armed Soldier > Bounce Diva back to hand for Birdman > Use Armed soldiers effect to normal summon Diva> Special summon another card which basically sets your field with
Birdman(3* tuner), Armed Soldier(2*), Diva(2* tuner), 4th obligatory card(level 2 or 3 monster)
Inherent summons, such as Swap Frog or Sea Lancer, do not work with Atlanteans, as they are not the actual effect of the monster (much like Cyber Dragon).
With the Genex Engine, you have access to some of the "Genex" synchros, Like Hydro Genex (Controller+WATER), Genex Ally Triarm (Controller+WATER to get the Spell/Trap destruction), or Genex Ally Axel to Special Summon a level 4 or lower Machine-type monster from the Graveyard).
With the Deep Sea Diva engine, and with Atlanteans being WATER, you also have access to some Synchros that require WATER-type monsters as material, like:
Dewloren, Tiger King of the Ice Barrier- Once per turn, you can Return any number of face-up cards you control to the owner's hand. For each card returned by this effect, this card gains 500 ATK until the End Phase.
Dewloren can be used to bounce Atlanteans back to the hand to resue their effects, or even cards like Dragon Ice and Mermail-Megaloabyss so that those card effects can be used again to send an Atlantean(s) to get their effect.
Gungnir, Dragon of the Ice Barrier- Once per turn: You can discard up to 2 cards to the Graveyard to target the same number of cards your opponent controls; destroy them.
With Brionac and Trishula Forbidden in Advanced Format, Atlanteans will turn to this card to discard for effect and do mass damage.
With Deep Sea Diva in the deck, it makes it easier to bring out more Atlanteans to Special Summon Poseidra Dragon, as well. While it takes 3 level 3 or lower WATER monsters to Special Summon Poseidra, it amy seem like it would be a one-shot deal. However, Atlanteans have their own Spell Card that helps with summoning the boss:
Target 3 level 3 or lower Sea-Serpent-type monsters in your graveyard, Special Summon those targets. You cannot Special Summon other monsters the turn you activate this card.
This almost works like a Scapegoat for Atlanteans; activate the card on your opponent's End Phase, and summon Poseidra on your turn, or Synchro/Xyz.
Other Spell/Trap tech that are useful to an Atlantean deck:
Bubble Bringer- Level 4 or higher monsters cannot attack directly. During your turn: you can send this face-up card from the field to the Graveyard to target 2 level 3 or lower WATER monsters with the same name in your Graveyard; Special Summon those targets, but their effects are negated.
Aquamirror Cycle- Target one face-up WATER monster you control and 2 WATER monsters in your Graveyard; shuffle the first target into the deck, and if you do, add the second targets to your hand.
Salvage- Add 2 WATER monsters with 1500 ATK or less from your Graveyard to your hand.
Surface- Target one level 3 or lower Dish, Sea Serpent, or Aqua-type monster in your Graveyard, Special Summon that target in face-up defense position.
Aegis of the Ocean Dragon Lord- Until the End Phase of this turn, face-up level 3 or lower WATER monsters you control cannot be destroyed by battle or card effects.
Abyss-Sphere- Special Summon one "Mermail" monster from your deck. It's efffects are negated. You cannot activate any Spell Cards. When this card leaves the field, destroy that monster. Destroy this card during your opponent's next End Phase after activation.
This is the backbone Trap Card for an Atlantean/Mermail hybrid deck. Can be searched by Mermail Abysspike.
Atlanteans are quickly becoming a powerful force. This is a thread to talk about engines, strategies, tech, and combos with this up and coming archetype!
THE OFFICIAL RELEASE DATE FOR THE STRUCTURE DECK IN THE TCG IS OCTOBER 16, 2012!!
*Thank you to Hellviper and Dudewitbow for contributing to the OP of this thread.