Ilbranteloth
Explorer
I don't think you need to have the rule about hitting with advantage or rolling a 20 to grapple. All of the creatures that have a grapple ability automatically do so if they hit. This is also in line with similar creatures. The escape DC should be 12 based on your stats, and the dissolve should be DC 10.
As for the dissolving itself, the Mummy Rot is a good model - the maximum hit point total is reduced once every 24 hours.
Here are the abilities using existing text from the MM.
Tentacle. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. If the target is Medium or smaller, it is grappled (escape DC 12) and restrained until the grapple ends. The tentamort has two tentacles, each of which can grapple one target. Each tentacle can be attacked (AC 20; 18 hit points, immunity to poison and psychic damage). Destroying a tentacle deals no damage to the tentamort. A severed tentacle regrows in 1d4 days.
Dissolve. The tentamort injects acid into targets grappled in its left tentacle through a bone needle causing 3 (1d6) acid damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or suffer its hit point maximum decreases by the same amount. The creature can't regain hit points, and its hit point maximum decreases by 3 (1d6) for every 24 hours that elapse. If the creature's hit point maximum is reduced to 0, the target dies, and its internal organs are dissolved into mush. The condition can be removed by a lesser restoration or greater cure disease magic.
Ilbranteloth
As for the dissolving itself, the Mummy Rot is a good model - the maximum hit point total is reduced once every 24 hours.
Here are the abilities using existing text from the MM.
Tentacle. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. If the target is Medium or smaller, it is grappled (escape DC 12) and restrained until the grapple ends. The tentamort has two tentacles, each of which can grapple one target. Each tentacle can be attacked (AC 20; 18 hit points, immunity to poison and psychic damage). Destroying a tentacle deals no damage to the tentamort. A severed tentacle regrows in 1d4 days.
Dissolve. The tentamort injects acid into targets grappled in its left tentacle through a bone needle causing 3 (1d6) acid damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or suffer its hit point maximum decreases by the same amount. The creature can't regain hit points, and its hit point maximum decreases by 3 (1d6) for every 24 hours that elapse. If the creature's hit point maximum is reduced to 0, the target dies, and its internal organs are dissolved into mush. The condition can be removed by a lesser restoration or greater cure disease magic.
Ilbranteloth
