Ancient Ascended. Aatrox has advantage on saving throws to resist spells and other magical effects. His attacks are considered magical for overcoming resistances and immunity.

Immutable Form. Aatrox is immune to any effect that would alter his form.

Sword-Bound. Aatrox cannot be disarmed of the Darkin Blade while he is alive.

Multiattack. Aatrox may use Infernal Chains if it is charged and take three greatsword attacks, or activate his Darkin Blade ability if it is recharged.

Greatsword. Melee Weapon Attack: [roll]1d20+17[/roll] to hit, reach 10 ft., 1 target. Hit: [roll]4d6+10[/roll] slashing damage and [roll]2d8[/roll] necrotic damage. Aatrox heals for half of the damage dealt.

Darkin Blade (Recharge 5-6 [roll]1d6[/roll]). Aatrox's greatsword burns with dark energy as he wields it with concussive force. He makes three attacks with this action, and cannot make any other actions afterward. He may move between these attacks.

• First attack. Aatrox damages a line 15 feet long and 10 feet wide in front of him. The adjacent spaces at the end of the area are the Critical Zone. All creatures in the area make a DC 22 Strength saving throw. On a failure, creatures in the lesser area take [roll]3d8[/roll] bludgeoning damage while creatures in the Critical Zone take [roll]4d8[/roll] bludgeoning damage and are knocked prone. Creatures that succeed take half damage and are not knocked prone.
• Second attack. Aatrox swipes his sword across an area in front of him. He damages a 40 feet rectangle in front of him; 10 feet away from him and 20 feet across. The spaces furthest from him is the Critical Zone. Creatures within the area make a DC 22 Dexterity saving throw. Creatures that fail within the lesser area take [roll]4d8[/roll] slashing damage, and [roll]6d8[/roll] slashing damage in the Critical Zone while also being knocked prone. Creatures that succeed take half damage and are not knocked prone.
• Third attack. Aatrox jumps into the air and slams his weapon to the ground with his full weight behind it. Creatures within a 20 feet radius, 10 feet tall cylinder centered on Aatrox makes a DC 22 Dexterity saving throw, taking [roll]6d8[/roll] bludgeoning damage on a failed save and half as much on a success. Creatures within 10 feet of Aatrox are in the Critical Zone. They take [roll]8d8[/roll] bludgeoning damage and are knocked prone on a failed save, and half as much damage and resist being knocked prone on a success.

Infernal Chains (Recharges 5-6 [roll]1d6[/roll]). Aatrox slams his Darkin Blade into the ground, creating a 10ft wide, 20ft long field of churning, chaotic energy that grasps for his enemies. Creatures caught in this field must make a DC 22 Strength saving throw or take [roll]4d8[/roll] bludgeoning damage and be yanked towards Aatrox, up to 10ft. Those who succeed their saving throw take half damage and are not pulled closer to Aatrox. The field then disappears.

Umbral Dash. As a bonus action, Aatrox can dash 15 feet forward. This can be used to move through terrain as Aatrox shifts through the planes. Aatrox cannot use this move if it would leave him inside of the terrain. 

(1/day) World Ender. When Aatrox reaches 0 hitpoints, he activates World Ender, gaining the following traits:

• All creatures who can see Aatrox within 60ft when this ability activates must make a DC 20 Wisdom Saving throw or be frightened of Aatrox for the next minute.
• Wide, leathery wings unfurl from his back, granting a flying speed equal to his walking speed.
• Can take an additional greatsword attack, both for actions and legendary actions. (4 greatsword attacks on his turn, can take 2 greatsword attacks with the Lengendary Attack action).
• World Ender stops at the end of Aatrox's next turn. World Ender can be extended for one additional round for each creature Aatrox knocks to 0 hitpoints while in World Ender form. 
• Aatrox does still regain hitpoints from his greatsword attacks in his World Ender form. When World Ender ends, he keeps these gained hitpoints.

Aatrox 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. Aatrox regains spent legendary actions at the start of his turn.

Attack. Aatrox can make one Greatsword attack at a creature within range.
Move. Aatrox can move up to his speed.
Umbral Dash (Costs 2 Actions). Can make one Umbral Dash in any direction. On his next turn, Aatrox cannot make an Umbral Dash.