Monarch of undeath. Krell cannot die in the normal sense. Instead, if Krell is reduced to 0 HP, he seems to be dead, but actually his spirit is transported to the necropolis to be put into a new body. In game terms, Krell always comes back unless his soul is destroyed.

To destroy Krell's soul, a creature must kill Krell with his own sword. Easier said than done.

Duelist. Melee attacks against Krell have a disadvantage. 

Undead king. All undead answer to krell, and do his bidding without question(even liches and red wizards).

Multiattack. Krell makes 3 attacks with the Blade of Krell. 1 of these attacks can be replaced with a Soul reap action. 

Soul reap. Krell attacks with the Blade of Krell, but with the following added:

• If the Unmodified roll for that attack was 15+, change the damage to 8d20+5, and the damage type to force
• If a creature is reduced to 0 HP by this attack, change their monster type to undead, and they become a Soulless minion of Krell
• In addition, the blade of Krell changes its damage to 8d20+2 for another 4 rounds if this minion dies

The Blade of Krell. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 45 (4d20 + .5) slashing damage. 

Krell can make 2 legendary actions at the end of his turn. These recharge every long rest.

Soul bind (Costs 2 Actions). Krell attempts to temporarily binds a creature's soul to the necropolis. That creature must make a DC 17 charisma save or attack its allies as if it were their enemy for the next round. 

Krell can take 3 mythic actions at the start of his turn. These recharge every long rest.

Wraith (Costs 3 Actions). Krell becomes a wraith, and can teleport to anywhere within 100 feet of him and in the same plane of existence. He can teleport a maximum of 3 times, and after that he becomes material again.