Crab Lord

Shapechanger. The crab lord can use its action to polymorph into the form of a humanoid or into its crab form. Its statistics are the same in each form. In its crab form it uses a claw attack and in in its humanoid form it uses a crush attack. Any equipment it carries is not transformed. If slain, the crab lord reverts to its crab form.
Amphibious. The crab lord can breathe air and water.
Magic Resistance. The crab lord has advantage on saving throws against spells and other magical effects. 
Succession. A destroyed crab lord will be succeeded by another crab in 24 hours. The successor becomes a crab lord and gains the memories of its predecessor. 
Spellcasting. The crab lord is a 13th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 17, +9 to hit with spell attacks). The crab lord has the following cleric spells prepared:
Cantrips (at will): spare the dying, thaumaturgy 
1st level (4 slots): command, sanctuary, shield of faith 
2nd level (3 slots): hold person, silence, spiritual weapon 
3rd level (3 slots): dispel magic, meld into stone
4th level (3 slots): divination, control water 
5th level (2 slots): dispel evil and good, geas 
6th level (1 slot): forbiddance
7th level (1 slot): plane shift

Multiattack. The crab lord makes one attack with its claw and one attack with its planar mancatcher.
Claw (Crush in Humanoid Form). Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 35 (9d6 + 4) bludgeoning damage and the target is grappled (escape DC 16). The crab lord has two claws (or two arms in humanoid form), but one is large and the other is small. Only the larger one can be used to attack or grapple.
Planar Mancatcher. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) piercing damage and the target is grappled (escape DC 16). The mancatcher can only be used to grapple small, medium or large creatures, and only one creature at a time. When a creature is grappled with a planar mancatcher it cannot be transported to another plane. If it tries to do so, it is inflicted with 1d10+4 magical damage and must succeed on a DC 16 Constitution save or become unconscious.

The crab lord can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The crab lord regains spent legendary actions at the start of its turn.
Attack. The crab lord makes one attack with its claw or planar mancatcher.
Blinding Dust. Blinding dust and sand swirls magically around the crab lord. Each creature within 5 feet of the crab lord must succeed on a DC 16 Constitution saving throw or be blinded until the end of the creature's next turn.
Whirlwind of Sand (Costs 2 Actions). The crab lord magically transforms into a whirlwind of sand, moves up to 60 feet, and reverts to its normal form. While in whirlwind form, the crab lord is immune to all damage, and it can't be grappled, petrified, knocked prone, restrained, or stunned. Equipment worn or carried by the crab lord remain in its possession.