Homebrew
Armed Defense. Sentron can use up to two of its remaining arms to guard the crack in its chest. For each arm protecting its chest, Sentron gains +2 to its AC. When Sentron is not guarding the hole in its chest, all damage immunities are lost except for poison and psychic damage. Arms actively guarding Sentron cannot be used to make attacks or perform any other action. Destructible Arms. Sentron has six arms, each with their own hit points and AC separate from Sentron itself and should be represented separately on the battlemap. Each arm has 17 (2d8+8) 2d8+8 hit points and 16 AC. They share their immunity to poison and psychic damage with Sentron, but do not benefit from Armed Defense. Once an arm reaches 0 hit points, it crumbles to the ground and cannot be used to make attacks or guard Sentron, preventing Sentron from using Multiattack once all but one arm has fallen. If Sentron loses all of its arms, it regenerates all of them at the start of its next turn. Mobile Transformation. Upon starting its turn with 0 hit points, Sentron will transform into its mobile form, regenerating all lost arms, pushing itself up from within the ground, and revealing a set of six, massive scimitars from its back.
Multiattack. Koloktos makes two returning double blade attacks or two slam attacks. Returning Double Blade. Ranged Weapon Attack: +3 1d20+3 to hit, range 45/90, one target. Hit: 6 (2d6-1) 2d6-1 slashing damage. Each creature in a line 5ft wide that extends out from Sentron to the target must make a DC 13 Dexterity saving throw, taking the damage dealt from the damage roll on a fail, whether or not the attack hits its intended target. Slam. Melee Weapon Attack: +11 1d20+11 to hit, reach 10ft, one target. Hit: 21 (3d8+7) 3d8+7 bludgeoning damage.
Setron can take 1 legendary action, choosing from the list below. Only one legenday action can be used at a time and only at the end of another creature’s turn. Setron regains spent legendary actions at the start of its turn. Summon Simbims. Sentron summons 0 (1d4-2) 1d4-2 Simbims into any unoccupied spaces within 80ft of itself that it can see.
Temfala Tower. Sentron's boss arena will contain six pillars situated around the edges of the room. If any one of Sentron's attacks make contact with one of said pillars, the pillar will crumble to the ground, potentially crushing any creatures caught underneath. All creatures within 10ft of a collapsing pillar must succeed on a DC 15 Dexterity saving throw or take 22 (4d10) 4d10 bludgeoning damage, halved on a success.