We're tackling another classic enemy, that somehow so many game designers have some kind of intense attraction for. Don't get me wrong, I see the appeal in making enemies out of mundane creatures that few would find threatening. I also see the fantasy in slaying a terrible enemy, only to eat their delicious insides as a mark of your victory. I've played Monster Hunter, and you can't tell me you wouldn't gobble up these Rathalos chicken tendies like they're on sale.
There is something about this one creature, though, that distorts creative minds and corrupts them, pushing them towards a state both vegetative and profound ; is Hidetaka Miyazaki responsible for this strange spell, or have I let the vapors of the poison swamp get to my head ? I'm not sure, so let's take a little trip to clear our heads. Why not to the beach ?
This is where our giant crab lives with its sycophants, and feasts on the foolish adventurers who dare disturb its stroll.
The thing with large beasts in D&D is that they tend to be quite straightforward. Usually relatively high on hit points, low in AC, with a couple attacks that they repeat round after round. If the DM is feeling spicy, the beast might stalk the party before attacking, or instead flee after being damaged, recovering to attack again. The problem, in both those cases, remains the same : beasts are overall boring to fight.
Due to their low intelligence, there's rarely many tactics they can use that would make sense, and the simplicity of their statblock allows for no special play from the DM, nor counterplay from the players.
For this statblock, I have focused on fixing these problems, so that this boss fight is truly a game of chess rather than a game of chicken. Let's take advantage of our beast's traits to make it powerful, but also give our players a chance to react to what it's doing, and play around it. As you've seen if you've read the previous post about our friend the Rollatinous Cube, I'm a big fan of alternate movement types. So let's crabwalk !
Arena and tactics
This crab looks tasty, but don't underestimate it : it's a true knight and warrior of the ocean !
This is a boss battle with minions : Clawdacious will summon some crab soldiers aka snapperlings at the start of each of its turns, surrounding its enemies. They are simple to run and have a single attack roll for each group attacking a different target.
Due to the nature of its body, the crab boss can mostly only move sideways, and cannot turn during its turn. Moreover, the crab attacks once with each pincer : the big pincer deals high damage and tosses enemies away, while the small pincer potentially poisons its target. However, each pincer can only attack one side of its body ! Since Clawdacious isn't the best tactician with its 2 intelligence, it will mostly be up to the party to optimize where they stand and move to avoid being hit by the claws.
The arena for this battle should contain pools of water through which the crab can move easily, but not the party. Geysers, hungry fish or other environmental threats are also great to add to the battlefield, since this one is all about positioning. Clawdacious can then use his Big Pincer attack to throw enemies into these hazards.
Be sure to explain the facing mechanic at the start of the combat. This is easy to keep track of both on VTTs and on your physical tokens : just have an arrow pointing somewhere on the token, and use that as the crab's current facing.
If you want to lean on the comical aspect of the battle, I'd recommend giving Clawdacious a boisterous personality, like an excessively proud knight. The smaller crabs can be sycophantic followers who seek protection, or perhaps even have their own, dark intentions...