Warden

Death Throes. When the warden dies, the neromatic aura of it can explode (Roll 1d10 on 6-10 it explodes), and each creature within 15 feet of it must make a DC 15 Dexterity saving throw, taking 30 necrotic damage on a failed save, or half as much damage on a successful one.

Necromancy Aura. At the start of each of the warden's turns, each creature within 5 feet of it takes 6 necrotic damage. A creature that touches the warden or hits it with a melee attack while within 10 feet of it takes 6 necrotic damage.

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

Magic Weapons. The warden’s weapon attacks are magical.

Regeneration. The warden regenerates 20 HP at the start of each of its turns unless it is hit with radiant energy. Then it regenerates only 10. With each regeneration, the Warden fuses more with its armor.

Multiattack. The warden makes two attacks: one with its Greataxe and one with the blade on a chain.

Greataxe. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 3d8 + 8 slashing damage plus 1d10 necrotic damage. If the Warden scores a critical hit, it rolls damage dice three times, instead of twice.

Barbed blade on a chain. DEX 15 saves from everyone within 15 ft. Hit: 2d6 + 8 slashing damage plus 3d6 necrotic damage on a failed save, half on a success. Those who fail the DEX save, must then succeed on a DC 15 Strength saving throw or be pulled up to 15 feet toward the Warden.

Has 2 reactions per turn.

Attack of Opportunity. If someone moves more than 10 ft. away from it, the Warden gets an attack of opportunity.

Miss stomp. If someone misses the Warden with a melee attack, the warden can stomp the target. The target needs to succeed on a DC 18 STR save or be knocked prone and take 3d6 bludgeoning damage.

Has 3 legendary actions.

Stomping charge. The warden charges in a straight line, stomping everyone in its path. Everyone in a 40 ft. line needs to make a DC 15 DEX saving throw or  take 3d6 bludgeoning damage. (roll a d6 to regain on a 5-6). The warden does not provoke attacks of opportunity this way.

Magic. It's eyes light up red and target within 60 ft. must succeed a DC 15 CHA saving throw or make one attack against an determined target within range.

Disarm. Within 10 ft. the warden chooses a target. The target has to make an 18 STR saving throw or be disarmed. The disarmed weapon then fuses with the Greataxe, creating a weapon monstrosity. (It requires an Athletics 18 check to free the weapon).