Alexander, Warden of Time (High Level Challenge)

Past, Present, Future. At the start of combat, Alexander establishes his domain over time, rolling for initiative thrice, occupying three places in the initiative order and taking three turns per round. Avatars of the Past and Future can redirect damage from Alexander and must be dispelled in order to allow the damage dealt to be actualized. Additionally, Alexander can maintain concentration on two spells at once.

Avatar of Past & Future HP Pools: 450 HP AC 20
Both Avatars will have identical actions to Alexander, Warden of Time but will not be able to use Temporal Stasis. Both Avatars can only be confronted in their perspective domains (Hereafter and Bygone Gaols)

(Avatar of Future) Tomorrow's Challenge. Damage dealt to Alexander while the Avatar of Future is active is absorbed by that Avatar up to a cumulative total of 450 HP. This reaction is inoperable if the Avatar is dispelled via direct intervention or by absorbing 450 points of damage or more.

(Avatar of Past) Footprints in Sand. Hostile creatures targeting Alexander while the Avatar of Past is active make wakes in the River of Time, allowing Alexander to see an impression of their intent. Declared attacks must be made with Disadvantage until the Avatar of the Past is dispelled via direct intervention.

Lord of Time. Alexander's Avatars of Past and Future must be pursued through time gates in order to eliminate them. Avatar of Past inhabits the Bygone Gaol plane and the Avatar of Future inhabits the Hereafter Gaol plane. Additionally, Alexander and his Avatars cannot be affected by manipulations of time unless they desire to be manipulated.

Legendary Resistance (3/Day). If Alexander fails a saving throw, he can choose to succeed instead.

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

Elder Primal. Alexander's aetherial density is that of an Elder Primal and cannot be affected by spells that alter his form or physical properties without his acceptance.

Ebb and Flow of Time (Top of Round) (Recharge 18-20) (3/Day). Alexander pulls at the River of Time, redirecting its flow. Select up to four targets and make competing saving throws utilizing their life's essence (Highest Stat) against a Constitution Roll made by Alexander. Adds or Subtracts 5 from the initiative rating of up to four targets within 75 ft. 

Spellcasting. Alexander is a 20th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 23, +11 to hit with spell attacks). Alexander has the following sorcerer spells prepared:
Cantrips (at will): Bolt of Time, Burden of Time, Resistance, Booming Blade
1st level (at will): Shield, Forced Time Loop
2nd level (at will): Mirror Image, Hold Person, Disrupt Time
3rd level (5 slots): Slow, Thunder Step, Alter Time
4th level (5 slots): Sickening Radiance
5th level (5 slots): Time Twist, Holy
6th level (4 slots): Argent's Investiture of Time
7th level (3 slots): Arrow of Time
8th level (2 slots): Rip Through Time
9th level (1 slot): Reverse Time

Multiattack. Alexander uses Temporal Stasis if it can. It then takes two follow-up actions.

Divine Strike. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 27 (4d12 + 10) bludgeoning damage and 6 (2d6) radiant damage. 

Dawn's Promise. Ranged Magic Attack: +11 to hit, range 50 ft., 3 targets. Hit: 11 (2d10 + 6) radiant damage to each target selected.

Temporal Stasis (Recharge 6). Alexander halts the flow of time for everyone but himself until the end of his turn. No time passes for other creatures, while Alexander can move and use actions as normal. Other creatures cannot take reactions in response to Alexander’s actions during this turn.

Cast to Futures Past (Recharge 4). Select one target to send to a moment in either the Past or Future. The target must make a Wisdom Saving Throw (DC 23), being cast to either the Hereafter Gaol or Bygone Gaol on a fail. Ability has no effect on a successful save.

Declaration of a Shared Sentence. Targets one creature to receive Divine Punishment, receiving 5d10 radiant damage that can be split evenly amongst allies within a 15 ft. radius at the start of their next turn. Reduce 1d10 for each additional target within range when the sentence is enacted, down to a minimum of 1d10.

Declaration of an Isolated Sentence. Targets up to five creatures to receive Divine Punishment, receiving 1d10 radiant damage in a 10 ft. radius at the start of their next turn. Close proximity to other creatures to receive punishment will result in an additional 1d10 of radiant damage being added to the damage received per additional ally within 10 ft.

Manifest Time Gates. Alexander twists time in two locations, opening portals bearing markings of the Future and Past. These portals will remain open for the remainder of the combat encounter, acting as access points to the Bygone and Hereafter Gaols.

Quickened Spell (2/Day). Alexander can use his Bonus Action to cast one spell up to Level 5 regardless of casting time.

Action Surge (2/Day). Alexander can reinvigorate his fighting prowess, allowing the execution of Divine Strike once more.

Bullet Time (8/Day). When an attack is declared, Alexander can choose to manipulate his internal time, allowing imperceptible movement to occur to try an evade an attack or spell. Roll a d20 and reduce damage accordingly:

1-9: Damage of Melee Attacks are reduced by 2d6. Attacks involving a saving throw deal 1/2 damage (Half if failed and 25% if success on the original saving throw).
10-18: Damage of Melee Attacks are reduced by 3d8. Attacks involving a saving throw deal 25% damage if the original roll was a failure or no damage on a success.
19: Damage of attacks are nullified.
20: The damage or spell is redirected back to the attacker at half potency, as time is selectively reversed.

Alexander, Warden of Time 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. Alexander regains spent Legendary Actions at the start of its turn.

Divine Strike (Costs 1 Actions). Alexander can perform his melee attack, with a bit of restraint, upon a hostile creature. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 17 (2d12 + 10) bludgeoning damage and 6 (2d6) radiant damage. 

Spawn Judgement Pentacle (Costs 2 Actions). Spawns 1d4-1 (1 minimum) mechanical orbs within one Domain of Alexander that will detonate for 2d8 Force Damage in a 25 ft. radius upon charging for 1 full round. Pentacles can be prematurely disarmed via aggressive action, leading to no explosion if successful.

Pentacles have an AC of 18 and 50 HP. They have no active attack, but will deal 1d4 force damage to all creatures that start their turn within 10 ft. of them or enter their field of influence.

Longinus Lances (Costs 2 Actions). Alexander materializes 3 lances of pure radiant energy to strike up to three hostile creature within 15 ft. of him. +9 to Hit. reach 15 ft., 3 targets. Hit: 9 (1d12 + 6) piercing damage and 6 (2d6) radiant damage per lance. After striking a target, the lances will remain as a spiritual weapon in a spot next to their original target with 35 HP and an AC of 18. The lances can attack as an action on subsequent turns after Alexander takes his turn, in rounds after the one in which they are summoned. This legendary action cannot be used while any Longinus Lance is currently active on the field.

Enigma Codex (Costs 3 Actions). Enter the description for your legendary action.

Alexander usurps the flow of time at certain breakpoints, enacting Divine Declarations as listed below.

Divine Judgement (Activated at 750 HP or Less). Alexander glows with a Divine Light as he channels his aether for a devastating assault. The attack is declared at the start of the round after the activation criteria is met, and will resolve at the conclusion of the round. When unleashed, radiant energy will explode from Alexander in an area within 60 ft. All hostile creatures in range must make a Dexterity Saving Throw (DC 26) and take 40d6 radiant damage, taking half as much on a successful save. Alexander is unable to use Legendary Actions or utilize Temporal Stasis upon the round this is executed, due to the aetherial strain of the attack.

Judgement Nisi (Activates on Round 3). At the start of the round, Alexander will make declarations of his current Judgement upon each living creature in his Domain. His decree will fill your mind as the voice of Alexander reads out your charges. Each person affected will be marked with a Nisi on their hand, either Past or Future, along with a 1 or 2, and must adhere to the edicts of the Judge within 2 rounds in order to avoid punishment. Failure to comply with one's judgement will result in 1d12 force damage per turn until the requirements of their Nisi has been satisfied.

Within each of the Gaols, Pentacles bearing the same markings and numbers will spawn. Pentacles can only be destroyed by the person ordained to be their destroyer, and are immune to attacks from anyone not bearing the same Nisi mark. Once the assigned pentacle is destroyed, the Nisi will be lifted and the person will be granted a blessing from Alexander, granting the use of one short-rest feature OR one additional spell slot up to level 6 as well as a heal of 30 HP.

Time Compression (Activated at 300HP or Less). Unconvinced of your current resolve to safeguard the Star, Alexander elects to compress all of time into a single moment to save all life from the Oblivion that awaits. Better to be in stasis and alive than allow the Star to become a lifeless void.

Once initiated, Alexander will stop acting and will instead begin to channel aether to enable the casting of one last spell. Time will crystalize upon Alexander, giving him a shield of 50 temporary hit points. Hostile creatures will have 2 rounds to disable the crystallized shield and deplete Alexander's remaining HP in order to interrupt this ritual and stop the compression of time. Failure to do so will result in all life being frozen in a temporal prison, unable to die but also unable to truly live.