Let's Build - Sandy Claws!
I tried my hand at the CR 10 version, but I don't have a good feel for it just yet. Take a look and let me know what you think.
Large beast, unaligned
Armor Class 18 (Natural Armor)
Hit Points 189 (18d10 + 90)
Speed 30 ft., swim 30 ft.
STR 21 (+5) DEX 15 (+2) CON 20 (+5) INT 9 (-1) WIS 12 (+1) CHA 9 (-1)
Saving Throws DEX +6, CON +9
Skills Stealth +4
Damage Resistances Fire
Damage Immunities Cold
Condition Immunities Charmed, Frightened, Grappled, Prone
Senses Blindsight 30 ft., Darkvision 60ft, Passive Perception 9
Challenge 10 (5,900 XP)
Amphibious. Sandy Claws can breathe air and water.
False Appearance. Until Sandy detaches itself from the wall of it's lair it is unnoticeable, save for the markings on its back resembling a large man with white hair in a red coat. It is covered with a thick layer of ice, further obscuring the markings at a distance
Multi-attack. Sandy makes two attacks with its claws
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 4 (2d6 + 5) bludgeoning damage, and the target is grappled (escape DC 14). Sandy has two claws, each of which can grapple only one target.
If a target is still grappled at the end of it's turn it takes an additional 1d6 fire damage from prolonged contact with the creature.
Chilling Gaze. Sandy looks into the eyes of a creature that it can see within 30 ft. That creature must make a Constitution saving throw (DC 15) or take 3d6 Cold Damage as a chill rushes through their body. The target is paralyzed until the end of Sandy's next turn as it is encased in a thin layer of ice.
Skitter (1 action). Sandy Claws can skitter sideways up to half its movement speed without provoking opportunity attacks
Better with Butter (1 action). Sandy makes one claw attack with an open claw, or repeats an attack against a grappled opponent if none are free.
If Looks Could Kill (2 actions). Sandy makes a Chilling Gaze attack against one opponent
Lair and Lair Actions
Baby It's Cold Outside. Every other creature within the lair must make a Constitution saving throw (DC 14) or start to be slowly encased in ice. They take 2d6 Cold Damage and are grappled by the ice. To escape they must break free (Strength check DC 16), or attack the ice. The ice has an AC of 10 and hit points equal to the damage taken.
Creatures still grappled in this way when this triggers again have disadvantage on the save, and take an additional d6 of Cold damage the hit points of the ice go up by the amount taken.
Chilled To The Bone. Any creature that has been hit with a Chilling Gaze attack takes 2d6 Necrotic Damage as Frostbite takes hold and must make a Wisdom saving throw (DC 14), if it fails the creature is frightened of Sandy Claws for 1 minute or until it makes the saving throw.