Fear Aura. Any non-Celestial creature that starts its turn within 20 feet of Aatrox must make a DC 23 Charisma saving throw, unless Aatrox is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to Aatrox's Fear Aura for 1 hour.

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

Darkin Weapon. Aatrox's weapon attacks are magical and, if disarmed, may call his weapon into his hand at the start of his turn. Aatrox is immune to any effect that would alter his form.

Preternatural Reflexes. Aatrox has two reactions and cannot be surprised without magical assistance.

Darkin Sight. Magical darkness doesn’t impede Aatrox’s darkvision.

Blood Well. Aatrox siphons blood from creatures he attacks and the fallen around him.
On a successful hit, Aatrox regains 9 (1d4 + 6) hit points; Aatrox can take the blood of creatures that have been dead no longer than 10 minutes (Undead, Constructs, Oozes, and some Elementals do not heal Aatrox as they have no blood).
As an action Aatrox can siphon the blood of all the fallen within 20 ft. of him healing 4 (1d6) per creature.
Once Aatrox has filled his Blood Well with 75 Hit Points worth of blood he may unleash his true power and form, World Ender.

Legendary Resistance (3/Long Rest). If Aatrox fails a saving throw, he can choose to succeed instead.

Behold My Curse (Mythic Trait; Recharges after a Short or Long Rest). If Aatrox is reduced to 0 hit points while in World Ender or within 3 rounds of its duration ending, he doesn’t die or fall unconscious. Instead, he kneels and siphons blood scattered around, regains 199 hit points, and attacks with all three strikes of Darkin Blade, prioritizing hitting the highest volume of creatures in range.

Multiattack. Aatrox can make 3 attacks with his Darkin Greatsword and targets 1 creature in range with Infernal Chains. If available Aatrox may replace one Darkin Greatsword attack with a Deathbringer Stance attack.

Darkin Greatsword. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 20 (3d6 + 8) slashing damage. World Ender dmg: 25 (4d6 + 9)

Deathbringer Stance (recharge on 4-6 or reducing a creature to 0 Hit Points). Deathbringer Stance doubles healing from Blood Well. A struck creature makes a DC 16 Constitution saving throw or suffer halved healing received for 1 minute. Affected creatures can 
Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 20 (3d6 + 8) piercing damage plus 12 (3d6) necrotic damage. World Ender dmg: 25 (4d6 + 9) plus 16 (4d6) necrotic damage.

Infernal Chains. Ranged Weapon Attack: +12 to hit, range 20/40, one target, 9 (1d4 + 6) piercing damage. Aatrox slams the ground with his fist. Spectral chains lunge toward a creature in range attempting to tether them to the ground.
On a hit, the area around the chain glows like hellfire; creating a perimeter of difficult terrain, centered on the target, that is 20 feet wide on the far end, 10 feet wide on the end closest to Aatrox, and 20 feet long. If the target hasn't left the perimeter by the start of Aatrox's next turn they will be pulled back to the center and the perimeter will disappear.

The Darkin Blade (Recharge 5-6, reducing a creature to 0 HP). As an action, Aatrox can unleash one of a three strike flurry that brings down his greatsword in a line, a cone, and a circle directly in front of him. Creatures at the tip of the first two and the center of the third must make DC 19 Dexterity saving throws or take increased damage and be knocked prone on a failure.

• First Strike. Attack: +15, 15-foot line. Hit: 28 (5d6 + 8) slashing damage. Tip Hit: 32 (6d6 + 8) slashing damage. World Ender dmg: 32 (6d6 + 9) / 40 (7d6 + 9) slashing damage.
• Second Strike. Attack: +15, 15-foot cone. Hit: 32 (6d6 + 8) slashing damage. Tip Hit: 38 (7d6 + 8) slashing damage. World Ender dmg: 38 (7d6 + 9) / 41 (8d6 + 9) slashing damage.
• Third Strike. Attack: +15, 10-foot radius circle. Hit: 38 (7d6 + 8) bludgeoning damage. Center Hit: 40 (8d6 + 8) bludgeoning damage. World Ender dmg: 41 (8d6 + 9) / 45 (9d6 + 9) bludgeoning damage.

World Ender (1/Short or Long Rest). Once the Blood Well has been filled, as an action, Aatrox can unleash his true demonic form for one minute, granting him several benefits. If Aatrox kills a creature during World Ender the duration is reset.

• Aatrox and his vessel roughly double in size, becoming a Gargantuan creature.
• Aatrox's Strength score increases to 28 (+9).
• Aatrox's attacks gain an extra 1d6 damage (see "World Ender dmg").
• Aatrox's Blood Well now heals him for 11 (1d10 + 5) hit points.
• Aatrox's walking speed increases by 10 and he gains a flying speed of 60.

Umbral Dash. Aatrox moves up to half his speed. Alternatively, Aatrox can reposition up to 10ft. in any direction while attacking with The Darkin Blade. This movement does not provoke attacks of opportunity.

God Slayer. If a creature casts a spell within melee range of Aatrox he may make an opportunity attack against it. If that creature is concentrating on a spell, it has disadvantage on its concentration check. Additionally if a creature is hit due to leaving Aatrox's melee range, their speed is reduced to 0.

Aatrox can take 3 legendary actions, using the options below. He can take only one legendary action at a time and only at the end of another creature’s turn. Aatrox regains spent legendary actions at the start of his turn.

Detect. Aatrox makes a Perception check.

Move. Aatrox moves up to half his speed.

Attack. Aatrox makes a Darkin Greatsword or Infernal Chains attack.

You Face a Darkin (Costs 2 Actions). Aatrox recharges then makes a Deathbringer Stance attack.

Rend (Costs 3 Actions). Aatrox recharges The Darkin Blade then uses the next strike in rotation.

If Aatrox’s mythic trait is active, he can use the options below as legendary actions for 1 hour after using Behold My Curse.

Cleave. Aatrox makes two attacks with Darkin Greatsword.

Chains of Oblivion (Costs 2 Actions). Aatrox sends Infernal Chains to all creatures in range.

Martial Prowess (Costs 2 Actions). Aatrox recharges The Darkin Blade then uses the strike after the last one used (1st->2nd->3rd->1st..).