A fiendishly influenced dire sword crab. Why?
1. It doesn't actually swim, so it's not limited to water.
2. It has native blindsight, so muddy, dark waters are its ally.
3. It can grab foes and drag them underwater to drown (if they can't breathe water).
4. Because it's a sword crab, it's logical for it to have some sort of Interception type ability to counter when someone strikes at it, maybe halving the damage.
5. It's fiendishly influenced because the cultists are evil, and this is a great excuse to make it resistant to fire and poison, and maybe acid, charm, and fear as well. It also has a slow regeneration (5hp/rd) as long as it's above 1hp, so if they just "chase it off," it'll be back.
Give it an AC of about 17 or 18 (natural armor), a speed of 40', and two attacks, each of which hit at about +10 for 2d10+6 damage (based on str 22), with a hit auto-grappling the target (escape DC 18). It can grapple up to two characters at a time, one in each claw. Subsequent attacks against a grappled target are made at advantage.
If it uses its Interception or Parry ability to reduce damage dealt by an attack it can see, it prefers to use a claw holding a grappled creature. That creature takes half the damage dealt by the attack, with the crab taking the other half.
Using the Blog of Holding method, it should have around 150-180hp as a solo monster. The damage is a bit below budget, but if it grabs a PC and starts using them to reduce damage from an attack every round, that'll up its effective DPR.
Be sure to use all the rules about underwater combat including reduced damage from fire spells, weapon restrictions, inability to cast spells with V components while underwater, etc.
