Croc, the Anti-Grappler - CR 11 Huge Beast

A few formatting notes:

• "Beast, Unaligned" should be lowercase
• "natrual" should be "natural"
• The average of 13d12 + 39 is 123, not 117
• Add a period after "ft" in Speed
• Damage Resistances should come before Condition Immunities
• Condition names should be lowercase
• Beasts don't usually have any supernatural abilities. Why can't this creature be tied up with chains (restrained) or flipped on its back (prone)?
• The order should be "bludgeoning, piercing, and slashing"
• "nonmagical" doesn't have a hyphen
• "sources" should be "attacks"
• Skills should come before DRs and CIs
• Stealth should be +3, not +5
• Tremorsense should be lowercase and should come before passive Perception
• Challenge is missing its XP value
• Croc is a unique creature, so what is Croc's biological sex? Use those pronouns throughout
• Legendary Resistance
• "day" should be capitalized
• The colon should be a period
• Wall Run
• The colon after the title should be bold
• Water Monster
• So while in water he can walk right through Bigby's Interposing Hand? That doesn't seem quite right. Also, anyone can swim through mud. Here's my suggested wording: "Croc ignores nonmagical difficult terrain while in water or mud."
• Multiattack
• Here's my suggested wording: "Croc makes two attacks, only one of which can be a bite. He can't make both attacks against the same target. Croc can use Roll in place of a bite attack."
• Bite
• "weapon attack" should be capitalized
• The average of 3d10 + 5 is 21, not 20
• Add spaces around "+" in damage calculations
• "targt" should be "target"
• Tail
• 14, not 13
• Roll
• There's no such thing as an Athletics saving throw. You mean a Strength saving throw.
• "they" should be "it"
• 16, not 15
• "and" should be "plus"
• Bludgeoning should come before piercing
• Add "as much" after "half"
• I would change "immediately" to "use its reaction to"
• This should be one Wide stat block, not three Thin stat blocks spread out
• Rolling Terror
• I strongly recommend making this roll's effects the same as the other roll. Otherwise it's kind of annoying for the DM.
• "attack of opportunity" should be "opportunity attack"
• What if the creature can't leap? Does it then not get that reaction?
• Also, you mention ending the grapple in two places, with success's effects spread across the description
• Here's my suggested implementation: "When a creature grapples Croc, he can use his Roll against that creature. If the creature succeeds on its saving throw, it can also use its reaction to move 5 feet away from Croc without provoking an opportunity attack from him, ending its grapple. If Croc is grappling a creature, that creature is also subjected to Croc's Roll."
• Escape Roll
• "actions" should be capitalized
• "save" should be "saving throw"
• 16, not 15
• "Bludgeoning" should be lowercase
• I would remove "and fall prone in Croc's space"

Your DCs are wrong. Bite's grapple should be 15, and every other should be 17, also, you should use Saving Throws for those features (except bite).
Also, it's pointless to make anti-grapple features, you already made it Huge, Medium creatures can't grapple it.

My group loves to grapple, and they're going to be trekking into a sewer system in the next session, so what better monster than a giant crocodile that hates being grabbed?

Croc won't be showing up alone - he'll be there with 4-8 of his smaller MM crocodile friends, and possibly also some giant rats. The encounter is set up in a large round room full of "Mud" (let's just call it mud...) where Croc can take advantage of his wall-run ability or dive into the murk if needed.

Croc is designed to isolate and grab one target and roll it to death, using his tail and his movement to keep other enemies at bay. Anyone who's ever seen a crocodile or alligator do their "Death roll" should have an idea of this guy's fighting style.