Axiomatic Mind. The decaton can't be compelled to act in a manner contrary to its nature or its instructions. 

Disintegration. If the decaton dies, its body disintegrates into dust, leaving behind its weapons and anything else it was carrying.

Innate Spellcasting. The decaton's innate spellcasting ability is wisdom (spell save DC 14) . It can innately cast the following spells, requiring no components:
At will: cure wounds,  shield
2/day each: heal, antilife shell

Multiattack. The decaton makes 10 tentacle attacks.

Tentacle. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 5 (1d8 + 1) bludgeoning damage. 

The decaton can take 2 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 decaton regains spent legendary actions at the start of its turn.

See. The decaton makes a perception check.

Tentacle. The decaton makes a tentacle attack.

Lumber. The decaton moves 10 feet.