This is the first article with the "Mazarbul Bestiary" tag. This series is pretty self-explanatory; I'll provide unique monsters, enemies, or NPCs for the various game systems that I cover. We'll kick things off with a custom creation for 13th Age.
I created the Iron Sea Lobster Behemoth to be a solo that isn't a solo. The main body of the creature is a Huge 5th level creature, but it has two claws that act as 4th level spoilers, as well as 6 tentacles that act as a group of 5th level mooks. So while it's mechanically multiple monsters, it's narratively just one. This solves the action economy problems of solos in D&D 4E, but it's admittedly not an elegant solution for every large creature. It works for a giant lobster because everybody's seen crabs walking around on the beach with missing claws, and it's feasible to say that even if the body is killed the claws might still reflexively attack (though I'd advise any GMs running this to just end the encounter when the body is killed, even if there are claws and/or tentacles left). Plus the tentacles are supposed to evoke a Cthulu-esque vibe. I certainly wouldn't use this option with a dragon, for example.
Iron Sea Lobster Behemoth Body
Huge 5th Level Troop Initiative +7
Fear Aura 24 HP
Stomping Scuttle + 10 vs AC (3 attacks) - 18 damage
R Acidic Brine (1d4 nearby enemies in a group) +10 vs PD - 15 acid damage and 5 ongoing.
Resist 16+ Fire
PD 19 HP 220
4th Level Spoiler Initiative +8
Special: the claws can intercept independently
Grabbing Claw +9 vs AC - 14 damage
Natural Even Hit: The target is grabbed (-5 to disengage checks unless the target hits the claw with an attack first, and the target cannot make opportunity attacks or ranged attacks)
PD 19 HP 56
5th Level Mook Initiative +8
Special: the tentacles can intercept independently
Stinging Grasp + 10 vs PD - 8 damage
16+: The target is also Dazed.
PD 18 HP 18, 36, 54, 72, 80, 98
By default an Iron Sea Lobster Behemoth is most likely going to be encountered on the rocky shores east of the Sea Wall. As a solo it's designed to work alone, and since it's a pretty mindless, hunger-driven monstrosity it's unlikely to have any allies. Perhaps it could pop up as a 3rd party during a conflict (drawn by the sounds of battle, or by the scent of blood). To increase the threat level have the encounter take place in shallow water (or have the tides rapidly flow in during the battle). This way the claws can hold a PC underwater after they're grabbed, changing the tactical landscape to "lop off that claw before Bob dies!" Also keep in mind the ability of the claws and tentacles to intercept melee PCs. The lobster behemoth will do all it can to protect its soft underbelly, and that means keeping potential threats at a distance (or at least Dazed). It will probably seek out the squishiest PC to make a meal of, softening them up with its acidic brine, but it won't foolishly consume an unconscious meal while being attacked (it may try to drag it underwater though!).
The Diabolist: While the origins of Iron Sea Lobster Behemoths are unknown (as is the case with most of the monsters that drag their hefty forms out of the Iron Sea), the tolerance that the creatures have for fire have led some to suspect that they're spawned in underwater Hellholes. The Diabolist probably knows whether or not that's true, but she's not letting anyone else in on her secret. Some whisper that that's because even though they're of demonic origin, she's failed to control them.
The High Druid: Her forces have been guarding the Dragon Empire (or perhaps more accurately, the Wild Wood) from anything that gets over the Sea Wall. That includes these creatures, if one were to ever venture far enough from the sea to be a threat. Oddly enough, smugglers have brought back rumors of druids and their agents going beyond the Sea Wall to take lobster behemoths out. Such tales lend credence to the "demonic origins" theories, or at the very least suggest that they're not natural. Or maybe the druids are harvesting body parts for some ritualistic reason...
The Lich King: Didn't expect his name on this list, did you? He definitely animates skeletons, and exoskeletons are skeletons too. He's the only icon to have exerted control over lobster behemoths, albeit after they were already dead.
Sea Wall Ruins - A scholar from Horizon has been reading up on the history of the ruins along the Sea Wall, and has reason to believe a legendary weapon lies there, after its wielder was slain defending the doomed fortress. What he didn't know was that the Iron Sea has recently eroded the shoreline right up to the walls of the fortress, which a lobster behemoth has claimed as its lair.
Quest for Caviar - A druid from the Blood Wood has put a call out to adventurers to collect lobster behemoth eggs. Several groups have already been sent out, but none have returned yet. Are the PCs hired by the druid to do what others could not, or by someone else who wants to see what the druid is up to? Does the High Druid herself even approve of this quest?
Origin Story - A hellhole near the Iron Sea (perhaps the mapped one south of the Spider Wood) has started emanating a briny, sea-air smell. Has it become flooded? Is there a lobster behemoth spawning there? Is the Lich King using this as a staging area for the creation of (exo)Skeleton minions? What does the Crusader have to say about the prospect of hellholes beneath the surface of the Iron Sea itself? These are the types of questions you'd have the luxury of asking if your pay grade was higher. Too bad you're just the guy they hired to scout it out.
Retrieval - Imperial soldiers patrolling the area outside of the fort on Cape Thunder were attacked by a lobster behemoth. One of them was killed, dragged underwater, and presumably eaten. Sad as that was, he happened to be carrying something very valuable that his companions would like back. They're sure as hell not going after the beast themselves, though.