Corrupted Razorclam

Buried in Sand. The razorclam is able to burrow itself just under the surface of a sandy seafloor giving itself advantage on Dexterity(Stealth) checks.

Razorshell. The razorclam has sharpened grooves all along its shell. Any creature that strikes it with a melee attack within 5 ft takes 2d4 slashing damage and must succeed on a DC13 Constitution saving throw or be diseased for 1d4+1 hours. After one round a diseased creature starts growing what appears to be gill structures and can only breathe underwater.

Multiattack. The razorclam makes one chomp and one pseudopod attack or two pseudopod attacks.

Chomp. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 27 (3d10 + 5) bludgeoning damage. On a hit, if the target is size medium or smaller it is pulled into the clam and considered restrained as the hard muscle and shell crush it. Any creature inside the clam takes 2d6+2 bludgeoning damage at the start of its turn.

Pseudopod. Ranged Weapon Attack: +5 to hit, range 15 ft., 1 target. Hit: 18 (2d10 + 3) bludgeoning damage.

Swiping Tentacles. Each creature within a 10 ft radius must succed on a DC 14 Strength saving throw as multiple tentacles shoot out of the shell battering everything around it. On a failure the creature takes 2d12 bludgeoning damage and half as much on a success.