Boundless Husk

Scorch Bound Weapon. Any weapon wield by Boundless Husk will be bound with it, granting it same raging flame(2d4 fire damage).
If killed in original state(in any other state weapon will turn to ash), scorch bond weapon can be obtain and wield, granting access to unique flame given by Boundless Husk as well granting wielder ability to be seen as another undead by all undead creatures, but weapon will bound with wielder making weapon unremovable unless wielder's hand is removed.

Boundless. Immune to turn undead, control undead, command undead any other spells allowing spellcaster to control undead's actions. (Does not affect spells such as command, hold person etc.)

One Goal Vengeance. Boundless Husks are driven by a singular purpose to kill their killer or any source of their undying rage. If their target is the necromancer who reanimated them, the Boundless Husk gains a advantage against them.

Ashes of Once Beating Heart. When Boundless Husk is killed in Semi Adapted or Fully Adapted State it will leave behind ashes of it's heart, which when consumed will grant fire immunity and increase of fire damage until long rest, after long rest person will engulf in flames getting 3d8 fire damage. Ashes can be as well used to turn regular undead into boundless husks. (If Boundless Husk never used Undying Heart, Set Ablaze Once More there will be no ashes)

Scorching Rage. The Boundless Husk does Barbarian Rage that increases fire damage as well(+3). 

Undying Heart, Set Ablaze Once More. Boundless Husk rips their undead heart dealing itself 1d20 Force Damage, but gaining ability to cast cantrip firebolt, control flame and message.

Scorched Cleave. The Boundless Husk swings it's weapon hitting multiple target, and setting nearby ground on fire for 3 turns.

Flaming Skin.  When someone attempts to grapple Boundless Husk they will get burn by their skin and dealing 1d3 fire damage.

Flaming Steps. With each step Boundless Husk leaves out flaming footsteps, if someone attempts to knock it prone, floor in 3 ft radius will turn into lava dealing 2d4 fire damage, lava will solidify after 2 turns.

Rage Beyond Death. When the Boundless Husk dies by damage type that is not fire or radiant it will go into cocoon state, in that state Boundless Husk is immobile and immune to all damage types but radiant which has resistance to in said state. If not destroyed during 5 turns cocoon will turn Boundless Husk into Semi Adapted state, which fully heals as well granting it 1-6 additional arms(depending on d6 roll) which provide additional attacks, if Boundless Husk not find any weapons nearby for new limbs it will do unarmed attacks. Semi Adapted state as well makes loose vulnerability to radiant damage, gain fire immunity as well resistance to all damage types used on it before turning into Semi Adapted state(with exception of bludgeoning, piercing and slashing damage).

Vengeance Without End. When Semi Adapted Boundless Husk dies to any damage type that is not radiant damage, it will go into cocoon state which has immunities to all damage types but radiant, unlike previous cocoon this one doesn't have resistance to radiant damage but will spill lava everywhere dealing 2d6 fire damage to anyone in 10 ft radius. If not destroyed within 10 turns it will turn into Full Adapted Boundless Husk, growing to large creature type increasing it hit points and restoring all of them, and gaining immunity to radiant damage, as well gaining resistances to bludgeoning, piercing, and slashing damage. But gaining vulnerability to psychic or force damage(depending on d20 roll, if it's 1-10 it's force damage, if 11-20 it's psychic) if Fully Adapted Boundless Husk is vulnerable to psychic damage it will weaken Boundless allowing spells that control undead's actions to bypass it, if roll successful crit.