Colossal Undead

Charge. If the Colossal Undead moves at least 15 ft straight towards its target and hits with a melee attack, the attack deals an additional 2d10 piercing damage.

Death Burst: When the Colossal Undead dies, it explodes, unleashing a wave of lightning and necrotic energy. Each creature withing 20 ft must make a DC 18 Dex saving throw, taking 4d6 lightning damage and 4d6 necrotic damage on a failed save, or half as much on a successful save. Creatures who fail the save are also knocked prone

Undead Aura: Each creature within 5 ft of the Colossal Undead takes 1d10 necrotic damage at the start of its turn. A creature that touches or hits it with a melee attack within 5 ft takes 1d6 lightning damage

Emissary of the Undead: All undead within 60ft of the Colossal Undead gain a +2 bonus to attack, damage and saving throws and have advantage against effects that turn them.

Magic Resistance: the Colossal Undead has advantage on saving throws against spells and other magical effects

Non Reactive: The Colossal Undead cannot use its reaction to make attacks of opportunity

Multiattack: The Colossal Undead can use Death Storm, then make 2 claw attacks. It can also use the bonus action to stomp on a creature that is prone

Claw: Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 2d6+5 Slashing damage and 1d6 Lightning damage. On hit the target must make a DC 15 Strength save or be knocked prone

Stomp: Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 2d10+5 Bludgeoning damage plus 2d6 Lightning Damage. On hit, the target must make a DC 13 Con save or become stunned

Death Storm: The Colossal Undead magically creates two bolts of necrotic energy, each of which can strike a target the Colossal Undead can see within 60 ft of it. The target must make a DC 15 Dex save, taking 2d10 Necrotic Damage on a failed save, or half as much on a successful one. 

Necroma Bolt. Once another undead creature within 60ft of the colossal undead has fallen, it absorbs their remains into itself and unleashes a flurry of black lightning. This does not use the Colossal Undeads reaction.

Ranged Spell attack: +7 to hit, range 60 ft, One target. Hit: 3d6 Lightning damage and 3d6 Necrotic Damage