Altered Earthswimmer

Creature Sense. The altered earthswimmer is aware of the presence of creatures within 10 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each creature’s Intelligence score, but can’t sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

Magic Resistance. The altered earthswimmer has advantage on saving throws against spells and other magical effects.

Tentacles. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) bludgeoning damage plus 11 (2d10) psychic damage. If the target is a Large or smaller creature, it must succeed on a DC 18 Strength saving throw or be swallowed by the altered earthswimmer. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the altered earthswimmer, and it takes 14 (4d6) acid damage at the start of each of the altered earthswimmer’s turns.

If the altered earthswimmer takes 20 damage or more on a single turn from a creature inside it, the altered earthswimmer must succeed on a DC 18 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the altered earthswimmer. If the altered earthswimmer dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Acid Breath (Recharge 5–6). The altered earthswimmer exhales acid in a 60-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 28 (8d6) acid damage on a failed save, or half as much damage on a successful one.

The altered earthswimmer has 3 legendary actions and can use them at the end of another creature's turn and recharge upon the beginning of its turn.

Telekinetic slam (1 action) - The target must make a DC 16 DEX save or be thrown 30 feet, if the target hits an object they stop and take 3D10 bludgeoning damage, decreasing by 1D10 per 10 feet travelled.

Rocky hide (1 action) - The earthswimmer uses its telekinetic abilities to cover itself in tough minerals and debris forming a thick shell. Until the end of its next turn it is immune to slashing and bludeoning damage but takes double damage from the shatter spell

Death knell (1 action, only usable when reduced to below 1hp) - The creature makes a final spiteful attack against the target that dealt the final blow against it. The target takes 19 (4d6+5) psychic damage and must make a DC 16 INT save. On a failed save the target has their INT, CHA and WIS halved, rounding down. The target can repeat the save once every 2 days.