Crota, Son of Oryx

Minion: At the start of combat, a Sword Bearer spawns in front of Crota. The Sword Bearer has it's turn before Crota. The Sword Bearer drops a Hive Sword when defeated.

Overshield: While active, Crota has a +3 to AC, resistance to all damage, and immunity to prone and stunned. After Crota takes 100 accumulative damage, the Overshield breaks. Crota loses his bonus to AC and resistances. Also, he is knocked prone, and becomes vulnerable to damage involving the Hive Sword. He regains his Overshield at the start of his next turn.

Reinforcements: When Crota's Overshield breaks, two Sword Bearers spawn next to him. They are added to the initiative when they spawn. If the Sword Bearers are defeated, they drop a Hive Sword. The Hive Sword stays out for two rounds, if no one picks it up. Only one Hive Sword can spawn at one time.

Hive Sword: The sword uses the stats of a Greatsword, and anyone wielding the Hive Sword has proficiency with it. The sword has 4 charges, and each attack costs 1 charge. You may expend all 4 charges to do an extra 2d8 damage.

Indestructible Soul: Crota dies if his health is dropped to 0 by the Hive Sword. If killed by any other source of damage, he immediately comes back up with 150hp and his Overshield.

Oversoul: If Crota does not die by the time he regains his Overshield for the fourth time, Crota unleashes an explosive wave of force that immediately kills the party.

Unending Nightmare: If the party is wiped by Oversoul, they return to the room before the encounter, and receive the benefits of a long rest. As Crota slowly crushes their will.

Legendary Resistance (3/Day). If Crota fails a saving throw, it can choose to succeed instead.

Spell-Resistant Carapace. While Overshield is active, Crota has advantage on saving throws against spells, and any creature that makes a spell attack against Crota has disadvantage on the attack roll.

Multiattack: Crota can make three attacks with either his Greatsword or Darkness Blast.

Greatsword. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 34 (8d6 + 10) force damage. 

Darkness Blast. Ranged Weapon Attack: +11 to hit, range 120/300 ft., one target. Hit: 32 (6d8 + 8) lightning damage.

Shadow Step. While in dim light or darkness, Crota can use his bonus action to teleport 60 ft into another area of dim light or darkness.

Counterattack. While Overshield is active, when Crota is hit with a melee attack, he can use his reaction to make one melee attack against the target.

Crota can take 3 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. Crota regains spent legendary actions at the start of its turn.

Greatsword (Costs 1 Action). Crota makes one Greatsword attack.

Shadow Step (Costs 2 Actions). Crota may use Shadow Step, however, the range is 30ft.

Slam (Costs 3 Actions). Crota slams the ground around him. Everyone within 10ft of Crota must succeed a DC 21 Strength saving throw. On a fail, the creature takes 6d10 force damage and is knocked back 30ft. On a success, the creature takes half the damage and only be knocked back 15ft.