Acnologia (Medium)

Legendary Resistance (5/Day). If Acnologia fails a saving throw, he can choose to succeed instead.

Time Theft. When a creature within 300 feet of Acnologia drops to 0 hit points, Acnologia can choose to siphon off some of its time, healing 55 (10d10 hit) points. If the amount healed surpasses his max HP, gain the excess as temporary hitpoints. 

Unbelievable Speed. Acnologia gains 2 additional turns in turn order. This second turn will always be at initiative count 16, the third at initiative count 9, and will not regain his legendary actions or reactions, but otherwise will act like a regular turn. 

Artifacts. Acnologia is seen with multiple magical items hovering around his body. They are as follows.

Penumbra's Mirror. Any magical effect that targets, whether directly or indirectly, Acnologia or the mirror itself, is absorbed and then recast back at the user. The mirror has an AC of 20 and 100 hitpoints. It is vulnerable to any physical attacks.

Antinogus's Breastplate. Any physical attack that targets, whether directly or indirectly, Acnologia or the armour itself, is absorbed and has its kinetic energy (damage) blasted back at the user. The armour has an AC of 20 and 100 hitpoints. It is vulnerable to any magical attacks.

Norvilles Mushroom Stew. At initiative count 20, the bowl will animate, feeding some stew to Acnologia, and healing him 33 (6d10) hitpoints. The bowl has an AC of 20 and 100 hitpoints. 

Atlas's Heart. At initiative count 19, this angelic heart orbitting around Acnologia will beat, causing everyone within 150 feet of it to take 16 (3D10) force damage. The heart has an AC of 20 and 100 hitpoints.

Multiattack. Acnologia makes blitz/shockwave impact attacks, choosing between how many of each he wants to use for a total of 3 attacks. Additionally, he can choose to use Frightful Presence.

Blitz. Melee Weapon Attack:  +15 to hit, Reach 15 Feet, one target, Hit: 21 (2d12 + 8) bludgeoning damage, and 16 (3d10) force damage. Acnologia launches devastating attacks targeting vitals at unseeable speeds. On a crit, it gives the target the stunned condition until the end of their next turn. 

 Shockwave Impact. Ranged Weapon Attack: + 15 to hit, reach 90 feet, one target, Hit: 21 (2d12 + 8) bludgeoning damage. Acnologia slams an open palm into the air with such speed that the air travels forward in a fist-shaped shockwave.

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the dragon’s Frightful Presence for the next 24 hours.

Time Breath (Recharge 5–6). The dragon exhales fire in a 30-foot cone. Each creature in that area must make a DC 20 Constitution saving throw, taking 54 (12d8) force damage on a failed save, or half as much damage on a successful one. On a failed save, the creature is magically weakened as it is desynchronized from the time stream.  While the creature is in this state, attack rolls against it have advantage, and the creature's movement speed is reduced to half, and it can't use reactions. On a successful save, a creature takes half as much damage only. A weakened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on 3 of these saves.

Time Skip. Acnologia warps time around him and appears somewhere he would be in another timeline. (Acnologia uses Dimension Door.)

Past of Pain. Acnologia mocks a character within 60 feet of him, insulting them, bringing up their worst pasts and awful futures. The character must succeed on a DC 20 WIS save or take 16 (3d10) psychic damage. Additionally, the character must remove 1d6 from their attack rolls, ability checks, and saving throws until the end of their next turn. On a success, take half damage and have no debuffs. 

Supreme Sentinel. Acnologia utilizes the same mechanics of the sentinel feat, with the additional benefit of hitting and stopping any and ALL movement, even if it specifies ignoring opportunity attacks. Additionally, Acnologia can use multi-attack instead of a singular attack. 

Timeline Shift. Acnologia changes the timeline in which an opponent's spell is cast to confuse and interrupt the weave. (Acnologia casts 6th-level counterspell)

Time Switch. In response to being targeted by a ranged attack or spell, Acnologia switches his position with another creature within 300 feet of him. The resulting attack or spell affects the other creature as if they were the target. Acnologia can also use this ability to evade melee attacks; however, in that case, the attacker can stop themselves from damaging the switched creature. 

Acnologia 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. The dragon regains spent legendary actions at the start of its turn.

Detect. Acnologia makes a Wisdom (Perception) check.

Blitz. Acnologia makes a blitz attack.

Past of Pain. Acnologia uses past of pain.

Maintenance. Acnologia repairs a non-destroyed artifact hovering around him for 25 hitpoints.

Form Switch [Mythic Only]. Acnologia switches to either Annihilation or Defence form. (Given he clears the correct conditions to enter said states.)

Upon being reduced to 0 hitpoints, all of Acnologia's magical items will fall to the ground and stop working. (Given they hadn't already been destroyed). Acnologia's hitpoints will regenerate to 250, he'll regain his legendary resistances, and he'll gain the following traits.

Defence Form. Acnologia enters a defensive state where he weaves through attacks by looking into the future and setting up time clones. In this state, Acnologia gains 3 reactions per turn order, resistance to all damage types, a save bonus of +10 on all his saves and upon taking damage from a melee attack, he will respond with one blitz attack of his own. In this form, Acnologia's speed is reduced to 20 feet, and he can't use his action to do anything but switch to Annihilation Form or dash. 

Annihilation Form. Acnologia enters this form when he sees an opening to take down a player. This can be due to a counterattack, bringing the player to below 20% HP, or it can be when a player is positioned too far from others to receive immediate help. It can be when Acnologia is rage-baited by a player, or simply when he sees fit. 

In Annihilation form, Acnologia retains his normal stats and abilities and will attack with the intent of knocking a player unconscious. While in this state, Acnologia's attacks automatically have advantage, and he crits on 17's and above. While in this state, he has vulnerability to all damage. If Acnologia is successful in downing a player on his turn, he will use his legendary action to switch back to Defence form at the end of his turn. If he's unable to down a player, he will instead be stuck in attack form until his next turn, where he can once more attempt to down a player to be able to switch back to defence form.