Ateniliosu, The Usurper

Legendary Resistance (4/Day). If Ateniliosu fails a saving throw, he can choose to succeed instead.

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

God of a Stolen Moment. Ateniliosu cannot be surprised and always acts on initiative count 20 unless another creature has a higher initiative result. Once per round when a creature within 120 feet rolls a natural 1 or natural 20 on an attack roll, saving throw, or ability check, Ateniliosu may force that creature to reroll the die and use the new result.

Temporal Instability. Ateniliosu exists slightly out of sync with the present. Opportunity attacks against him are made with disadvantage, and whenever a creature misses him with an attack, Ateniliosu may move 10 feet without provoking opportunity attacks. This movement does not require a reaction.

Concentrated Divinity. Ateniliosu's spell attacks and magical effects count as magical and ignore resistance to fire and radiant damage, though immunity still applies normally.

Collapse of the False God. When Ateniliosu is reduced to 0 hit points, excess damage does not carry over into another form. His divine shell fractures and disperses, ending this stat block and triggering his next form.

Ateniliosu is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). He requires no material components.

At Will: Fire Bolt, Sacred Flame, Thaumaturgy, Misty Step, Scorching Ray
3/Day Each: Counterspell, Dispel Magic, Fireball, Haste, Slow, Dimension Door, Wall of Fire
2/Day Each: Flame Strike, Hold Monster, Telekinesis, Sunbeam
1/Day Each: Disintegrate, Fire Storm, Reverse Gravity, Time Stop

I would be careful with Time Stop mechanically. I would use it mostly for repositioning, setting hazards, or creating visual chaos rather than unloading unavoidable damage.

Multiattack. Ateniliosu uses Solar Bolt twice and may cast one spell of 5th level or lower.

Solar Bolt. Ranged Spell Attack: +16 to hit, range 180 ft., one target. Hit: 24 (4d8 + 6) radiant damage plus 11 (2d10) fire damage.

Temporal Burn. Ateniliosu chooses one creature he can see within 120 feet. The target must make a DC 23 Constitution saving throw. On a failed save, the creature takes 36 (8d8) force damage and cannot take reactions until the end of its next turn. On a successful save, the creature takes half as much damage.

Time Skip (Recharge 5–6). Ateniliosu disappears from the current moment and reappears up to 120 feet away at a point he can see. He then immediately takes one additional action that can be used only to cast a spell, use Solar Bolt, Dash, or Disengage.

Rewind Wound (Recharge 6). Ateniliosu restores himself to the physical state he occupied at the beginning of his previous turn. He regains hit points equal to the amount of damage he has taken since then, up to a maximum of 50 hit points. This cannot raise him above his current phase's maximum hit points.

Accelerated Decay (Recharge 5–6). Ateniliosu targets one creature within 90 feet. The target must make a DC 23 Constitution saving throw, taking 45 (10d8) necrotic damage on a failed save or half as much on a successful one. On a failed save, the target's speed is also halved until the end of its next turn as its body momentarily ages.

Solar Detonation (Recharge 6). Ateniliosu creates a sphere of condensed fire and radiant energy at a point within 150 feet. Each creature within 20 feet of that point must make a DC 23 Dexterity saving throw, taking 49 (11d8) fire and radiant damage on a failed save or half as much on a successful one.

Blink Through Time. Ateniliosu teleports up to 60 feet to an unoccupied space he can see. This movement does not provoke opportunity attacks.

Accelerate. Ateniliosu chooses himself or one creature he can see within 60 feet. Until the beginning of Ateniliosu's next turn, that creature's speed increases by 20 feet and it gains a +2 bonus to AC.

Delay. Ateniliosu chooses one creature he can see within 60 feet. The target must succeed on a DC 23 Wisdom saving throw or have its initiative count reduced by 5 until the end of the next round.

Not Yet. When Ateniliosu would be hit by an attack, he can add +4 to his AC against that attack, potentially causing it to miss.

Rewind the Strike. When Ateniliosu takes damage from a single attack, he can reduce the damage by 20 and teleport up to 20 feet.

Counterspell. Ateniliosu casts Counterspell without expending a daily use.

Ateniliosu can take 5 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. Ateniliosu regains spent legendary actions at the start of his turn.

Blink. Ateniliosu teleports up to 30 feet to a space he can see.

Solar Bolt. Ateniliosu makes one Solar Bolt attack.

Stolen Second. Ateniliosu chooses one creature within 60 feet. The creature cannot take reactions until the beginning of its next turn.

Temporal Shift (Costs 2 Actions). Ateniliosu teleports up to 60 feet and may make one Solar Bolt attack before or after teleporting.

Burning Epoch (Costs 2 Actions). Ateniliosu chooses one point within 90 feet. Each creature within 10 feet of that point must make a DC 22 Dexterity saving throw or take 18 (4d8) fire damage.

Reorder the Moment (Costs 3 Actions). Ateniliosu chooses two creatures he can see. Their initiative positions are swapped until the end of the next round.

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Mythic Action Name (Costs # Actions). Enter the description for your mythic action.