The 21st Main Box Cybernetic Rebellion is bringing exactly Mermails have been looking for since their introduction in Duel Links: a Turn 1 play. Mermail Abysslinde and Abyss-sphere are two incredible cards that allow you to Special Summon any “Mermail” monster directly from your Deck, giving you direct access to anything you need.
Use Abyss-sphere to Special Summon Mermail Abysslinde. If Abyss-sphere leaves the field, Abysslinde will be destroyed and it will allow you to Special Summon a “Mermail” monster from your Deck. Even if nothing happens to Abyss-sphere, during the next End Phase of your opponent’s turn after its activation, the Continuous Trap will destroy itself.
Keep in mind that, if your opponent responds to the activation of Abyss-sphere by activating Cosmic Cyclone, banishing the Continuous Trap, you don’t get to Special Summon any monster.
You can use Mermail Abysslinde’s effect to Special Summon Mermail Abysspike and then use Abysspike’s effect to add a Level 3 WATER monster from your Deck to your hand.
If you activate Abysspike’s effect by discarding Atlantean Marksman, you will get to the target and destroy a face-up card your opponent controls.
Keep in mind that, if you target a face-up “Subterror” monster and your opponent response by activating Subterror Final Battle and changing that monster to face-down Defense Position, the monster will not be destroyed because it’s not a suitable target anymore.
If you activate Abysspike’s effect by discarding Atlantean Heavy Infantry, you will get to the target and destroy a face-down card your opponent controls.
Keep in mind that, if you target a face-down Spell/Trap Card and your opponent activates it in response, the card will not be destroyed because it’s not a suitable target anymore.
If you activate Abysspike’s effect by discarding Mermail Abyssgunde, it will allow you to target a “Mermail” monster in your Graveyard, except Mermail Abyssgunde, and Special Summon it.
You can just Normal Summon Mermail Abysspike, discard Abyssgunde, Special Summon Mermail Abyssmegalo from your Graveyard and then utilize Abysspike to activate its effect.
If you want the monster Special Summoned by Abyss-sphere to stay on the field, you can activate it during your opponent’s End Phase.
You could also use the Continuous Trap to Special Summon Mermail Abyssmegalo or Mermail Abyssteus, but you wouldn’t get much value out of them since even Dragon Spirit of White and Legendary Six Samurai - Shi En are able to get over them without Beatdown! or The Tie that Binds. If you’re sure your opponent is not going to be able to remove it from the field, you can use Abysslinde’s effect to Special Summon Mermail Abyssmegalo directly from your Deck. You will not get to search an “Abyss-” Spell/Trap Card if you don’t Special Summon Abyssmegalo via its own effect, but you’re still getting access to your main monster with relative ease.
You can Special Summon Mermail Abyssmegalo from your hand by discarding two WATER monsters. By discarding Atlantean Marksman, Heavy Infantry or Mermail Abyssgunde, you can get back some value from the resources you’ve invested to Summon Abyssmegalo.
When it’s Special Summoned by its own effect, Mermail Abyssmegalo allows you to add an “Abyss-” Spell/Trap Card from your Deck to your hand.
Abyss-scale of Cetus can be sent to the Graveyard to negate the effect of an opponent’s Trap Card;
Abyss-scale of Mizuchi can be sent to the Graveyard to negate the effect of an opponent’s Spell Card;
Abyss-scale of the Kraken can be sent to the Graveyard to negate the effect of an opponent’s monster.
Alternatively, you can use Abyssmegalo’s effect to search Abyss-sphere, which I’ve already talked about.
Unlike Abyssmegalo, Abyssteus requires you to discard only one WATER monster in order to be Summoned and, to make it even more cost-efficient, it also allows you to add a Level 4 or lower “Mermail” monster from your Deck to your hand when it’s Summoned this way.
Sealed Tombs Until the end of the opponent's next turn, neither player can banish cards from the Graveyard or Special Summon monsters from the Graveyard. This skill can only be used once per Duel.
Abyssmegalo and Abyssteus have a pretty lackluster ATK, so, even if you’re not going to be able to get them on the field together that often, it’s nice to have a way of getting over monsters like Dragon Spirit of White and Shi En without having to rely on an Equip Spell, which can be negated or banished. Ideally, you would get rid of your opponent’s monsters via Heavy Infantry’s effect and then OTK them, but that’s not always going to be possible.
Mermails are still quite fragile and most of their plays end up consuming all of their resources, which means cards like Drowning Mirror Force and Wall of Disruption can be devastating.
Atlantean Marksman will often not be enough to get rid of your opponent’s entire back row, but there’s also not enough space for Hey, Trunade! or Cosmic Cyclone.
Subterror Behemoth Umastryx can’t be destroyed by Abyssmegalo without an Equip Spell or Beatdown and at least another Level 5 or higher monster, Subterror Final Battle is nearly immune to Atlantean Marksman’s effect and your opponent can banish Abyssmegalo before you even get a chance to equip Abyss-scale of Cetus to it.
I've got all mermails from Gaia Genesis and played lots of pvp duels with it, and in my opinion the only mermail deck with some potential in the current meta is, still, the abyssbalaen OTK, as it can basically wipe out the entire board while summoning 2 or 3 big monsters. Nonetheless, the deck suffers from consistency issues and is also very risky, as you have to discard 4 cards to summon abyssbalaen, so if you can't otk your opponent for any reason and he manages to destroy your monsters on his next turn, your comeback ability is basically unexistent (although the same could be said about other decks, like AG). That being said, when the XYZ era comes, this deck will really shine, what makes me think that maybe we are getting the Zexal world still this year, as this new box brought mermail support and finally brought cyber dragon, two really strong archetypes in the XYZ engine.