A living sea mine in the form of a mechanical pufferfish/blowfish. Could also be a huge pufferfish/blowfish that swallowed a sea mine for a more whimsical theme.
Pufferfish:
Sea mine:
Pufferfish that ate a sea mine/mechanical pufferfish:
The more I think about it, the better I like the idea of a real pufferfish eating a sea mine! 
Could add an eyepatch too for style points.
Art presented is not my own and merely used as a concept
The story of the Antikythera Mechanism is one of my favourites of all time. For those that don’t know, a clockwork machine dating from about 150 BC and originating from Ancient Greece was discovered in a shipwreck off the Greek island Antikythera.
It was discovered at the turn of the 20th century but not worked on in earnest until the 50s, when it took almost 25 years for academics to figure out its purpose. Essentially it’s an analogue clock/calendar/zodiac calculator that demonstrates the maker’s deep understanding of mathematics, astronomy, and engineering. Made by a single person, it shows historians that Ancient Greece was more advanced than we had realised, that artisans also could be learned in science, and that Babylonian astronomers were keeping records of the stars which allowed the Antikythera Mechanism to be constructed to correctly predict the zodiac phase on any date with such accuracy.
Building on that, I want to see a clockwork mechanism from a shipwreck that was either recovered by or fused with a merfolk. The mechanism is of unknown origin and shouldn’t invoke Adana too heavily.
The mech part is based on ship navigation - think sextants, quadrants, compasses, telescopes, backstaffs, and nocturnals - and gives its merfolk wielder deadly accuracy. To my mind, this troop is a merfolk assassin; not rogue-ish in personality, but a deadly killer. True damage, single target, suberge and stealthy would all be appropriate. Clockwork should be clean (not rusted, it’s still working), but green due to iodising.
Mech inspiration palette:
I think it’s worth remembering that Merlantis is out the end of the month. So theme/visually it needs to be distinct and different enough for it to be from Blackhawk and not from Merlantis.
I would go with a mermaid that has had cybernetic enhancements to both walk and be able to breathe air. Steampunk/horror inspired and not very attractive as a result of her modifications. She carries a large electrified spear.
“Marma was a normal mermaid until the dreaded Kraken, blown off course by a terrible storm, attacked her home in Merlantis. Her family killed before her eyes, Marma swore revenge. After striking a deal with Sparkgrinder, Marma had her body transformed into a living weapon. She now spends her days in the kingdom of Blackhawk, waiting for the Kraken to show itself so she can finally have her revenge.”
EDIT: Not my artwork but something akin to this, with legs?
One of my favorite prehistoric fish is the Dunkleosteus. While Dunkleosteus was outlasted by sharks on the evolutionary ladder, I think it’s strength and resilience would be respected and feared by Merfolk.
“Nah, I don’t need a harpoon, I’ve got a Dunk.”
A fully mechanized, say 8’ tall, bipedal Dunkleosteus. Head almost straight up Dunkleosteus, perhaps even a special fossilized or mineralized skull of a Dunkleosteus with a mechanical body, inspired by DRACOS 1337.
Blackhawk = pirate kingdom
Mech = robot
Merman = aquatic
I just see him/her/it as the Minotaur version of the sea. Literally half human and half robot. Only the head is human while the rest is a synthetic body capable of swimming the deep blue sea with a triple barrel cannon arm in one hand and a special magic brew of rum in the other hand. Excuse me a moment while I daydream about this one…
Okay, make that a long while… 
old, tired, whale based merfolk,
with a lot of part mechanised due to old battle injuries
he could have some mechanical additions, weird armos or even whole vehicle
A whale merfolk with a spinner in his hand? Days best thats for sure!
One issue with this process is that certain troops appearance hamper the design later… whales will likely have devour, a turtle likely being a tank etc.
Nice sketch, @Skarbourne 
Might be a good mana creator for black hawk.
I am thinking a Electric Angler Fish. With a entrapment feeling.
I LOVE THE BORG steampunk…WICKED!!!
A mermaid warrior with a spider-leg mechanical apparatus to walk on land.
Lots of dariusburst chronicle saviours inspiration here. I like it.
Got
To
Be
THIS!
I was thinking about an Orca with mechanical enhancements. It got tired of Johnny Bronze poaching his meals so decided to take matters into Its own flippers.
It’s spell would be called Sonic Cutter. Maybe a new status effect called Deafness where a random troop lost its next turn (meaning the troop would grey out so you can’t select it)
It would also unfreeze any troop on the board, but burn them instead. At least it would counter mab and frozen (since merfolk wouldn’t like ice) It would do some kinda of spread damage based on allied mech/merfolk but not sure exactly how to scale it. It would be a blue/yellow troop.
any more insight on how that orca/mech would look like?
Considering Orcas have no natural predators I feel that their only beef would be against humans so they’ve developed technology with the help of the merfolk. It would be more like a cybernetic suit.
I found these images online so disclaimer they are not my ideas but the ones of the respective artist.
I’d envision more of an anthromorphic style orca, but not to the point they’ve grown legs. The cybernetics would have legs and arms so he could fight on land. They’re still orcas with a bone to pick with Johnny Bronze. They’re definitely “Chinooked” off with him!
