Altellax, the Void Corrupted Archmage

Entrenching Tentacles. When Altellax uses Spawn Tentacle and the Spawned Tentacle(s) are still active; Altellax's movement speed becomes 0, but he cannot forcefully moved. Gains Tremorsense (100 ft.).

Eyes of the Void. Altellax has Darkvision (200 ft.) and Truesight (150 ft.).

Grappling Tendrils. Altellax can Grapple up to five creatures at a time.

Legendary Resistance (5/days). If Altellax fails a saving throw, it can choose to succeed instead.

Return to the Void. When Altellax's Hit Points becomes 0; he is shifted into the plane of the Void, and remain there for 100 (10d10 +10) days as he regenerates his body.

Spellcasting. Altellax is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 23, +16 to hit with spell attacks). Altellac has following Wizard spells prepared:

• Cantrips (at will): Blade Ward, Firebolt, Frostbite, Mending, Message, Mind Sliver, Sapping Sting, Shocking Grasp, Thunderclap
• 1st level (4 slots): Absorb Elements, Detect Magic, Ray of Sickness, Shield, Thunderwave
• 2nd level (4 slots): Aganazzar's Scorcher, Arcane Lock, Blindness/Deafness, Detect Thoughts, Dragon's Breath, Gust of Wind, Hold Person, Mind Spike,  Ray of Enfeeblement, Spider Climb, Suggestion, Vortex Warp
• 3rd level (4 slots): Bestow Curse, Counterspell, Dispel Magic, Fear, Fly, Glyph of Warding, Haste, Lightning Bolt, Pulse Wave, Remove Curse, Sending, Vampiric Touch
• 4th level (3 slots): Banishment, Dimension Door, Fabricate, Fire Shield, Gravity Sinkhole, Ice Storm, Locate Creature
• 5th level (3 slots): Cone of Cold, Control Winds, Dominate Person, Hold Monster, Modify Memory, Negative Energy Flood, Planar Binding, Scrying, Telekinesis, Wall of Force
• 6th level (3 slots): Chain Lightning, Disintegrate, Eyebite, Mental Prison, Soul Cage
• 7th level (2 slots): Crown of Stars, Delayed Blast Fireball, Dream of the Blue Veil, Finger of Death, Plane Shift, Teleport
• 8th level (2 slots): Dark Star, Dominate Monster, Feeblemind, Maze
• 9th level (1 slots): Gate, Imprisonment, Psychic Scream, Ravenous Void, Time Stop

Void Absorption. Whenever Altellax takes Void Damage, Altellax regains Hit Points instead.  

Void Leak. Void Leak can be placed four times on the same creature at a time. If a creature is afflicted with Void Leak, whenever they're hit by Altellax's or Spawned Tentacle's attacks, spells, or any abilities that doesn't place Void Leak on the creature. Deal an additional d10 +5 Void Damage pre Void Leak on the creature. After damage is dealt; in a 10 ft. sphere with the afflicted creature as the center. All creature(s) in the affected area must make DC 22 Dexterity Saving throw, on success half the damage. On failed the creature(s) take same amount of Void damage as the afflicted creature. Then remove Void Leak from the afflicted creature. Void Leak is removed when Altellax is incapacitated, killed, or is no longer in the same plane as the afflicted creature(s).

Void Overwhelming (Mythic Trait: Recharges after a Short Rest of Long Rest). After drawing too much power from the Void, Altellax's body is overwhelmed and large dark eyeballs start to erupt through his body. Four eyeballs rip through his chest, and two eyeballs rip through his abdomen. When Altellax gain 200 hit points from the effect of Void Absorption, he can use a Reaction to activate this trait. This trait becomes inactive when Altellax is incapacitated. 

Multi Attack. Make four attacks with Tentacle or Void Staff. Or two Void Bolt

Tentacle. Melee Weapon Attack: +11 to hit, reach 15 ft., 1 target. Hit: 16 (2d10 + 6) Bludgeoning damage. After dealing damage, the creature must make a DC 22 Strength Saving throw, on fail the creature is pushed back by 10 ft. Or the creature makes a DC 20 Dexterity Saving throw, on fail the creature is then Grappled and then pulled to be within 5 ft. of Altellax. A creature(s) Grappled can repeat the saving throw at the start of their turn.

Spawn Tentacle. Summon a Spawned Tentacle that is within 30 ft. of Altellax. The Spawned Tentacle(s) do not have their own turn in the turn order. The Spawned Tentacle takes up that space equal to it's size (Medium). If the Spawned Tentacle is summon on the same space as another medium, small, or tiny creature. That must make a DC 22 Dexterity Saving throw, on success the creature moves 5 ft. away from the place they were. On fail the creature is now Grappled by the Spawned Tentacle. Altellax can only have four Spawned Tentacle out at a time. Can use a Bonus Action to dismiss as many Spawned Tentacle. If Altellax teleports or shifts to another plane, all Spawn Tentacle(s) are immediately dismissed. If a total of eight Spawned Tentacle are dismiss or killed, Altellax cannot use this Action/Legendary Action anymore until Altellax finishes a short rest. (Look at Altellax's Spawned Tentacle for stats).

Void Staff. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 12 (2d8 +6) Bludgeoning damage and 15 (1d8 +10) Cold Damage. After dealing damage, the creature must make a DC 22 Constitution Saving throw. On fail the creature is afflicted with Void Leak.

Void Bolt. Ranged Spell Attack: +16 to hit, range 100 ft., 1 target. Hit: 15 (1d8 +10) Cold Damage. After dealing damage, the creature must make a DC 22 Constitution Saving throw. On fail the creature is afflicted with Void Leak.

Void Ray (Recharge 4-6). 100 ft. line; All creatures in the affected area must make a DC 22 Dexterity Saving throw; on success, half damage. Deal 50 (10d8 +10) Cold Damage. After the creature(s) take damage they must make a DC 22 Constitution Saving throw, on fail the creature(s) gets 2 Void Leaks on them. Creature's Void Leak can not go over the maximum amount.

Void Wave (Recharge 5-6). 15 ft. sphere with Altellax and all Spawned Tentacle(s) as the center; All creatures in the affected area must make a DC 22 Dexterity Saving throw; on success, half damage. Deal 80 (12d8 +10) Cold Damage. After the creature(s) take damage they must make a DC 22 Constitution Saving throw, on fail the creature(s) gets 3 Void Leaks on them. Creature's Void Leak can not go over the maximum amount.

Command the Spawn Tentacles. Can only use this Bonus Action if Altellax have at least one Spawned Tentacle active; have all Spawned Tentacle(s) make one action, or use their bonus action.

Rearrange Tentacle. Can only use this Bonus Action if Altellax have at least one Spawned Tentacle active; Altellax can choice one Spawned Tentacle and move it to another location as long as it within Spawn Tentacle range. If the Spawned Tentacle is summon on the same space as another medium, small, or tiny creature. That must make a DC 22 Dexterity Saving throw, on success the creature moves 5 ft. away from the place they were. On fail the creature is now Grappled by the Spawned Tentacle. Altellax cannot choice a Spawned Tentacle that was summoned that round.

Void Retaliation. When a melee weapon attack is made against Atellax, use a Reaction to make the melee weapon attack miss. Then make one Void Staff attack towards the creature that made the melee weapon attack, if the creature is within the range of Void Staff. If the creature fail their saving throw for this attack, they're also Stunned; they can repeat the saving throw at the end of their turn. The creature is no longer Stunned at the start of Atellax's turn.

Void Reflection. When a ranged spell attack is made against Altellax, use a Reaction to make the ranged spell attack miss. Then make one Void Bolt towards to the creature that made the ranged spell attack, If the creature is within range of Void Bolt. The damage is increased by 1d4 on this attack equal to the attacker's spellcasting level. 5th level (2d4), 11th level (3d4), 17th level (4d4)

Altellax 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. Altellax regains spent legendary actions at the start of its turn.

Cast Cantrip (Costs 1 Action). Cast one Cantrip.

Tentacle Spawn (Costs 1 Action). Use Spawn Tentacle.

Cast Lesser Leveled Spell (Costs 2 Actions). Cast one spell between 1st and 3th level. The spell chosen cannot be casted at a higher level.

Crushing Grip (Costs 2 Actions). If Altellax has a creature(s) Grappled, to deal 16 (2d10+6) Bludgeoning to the Grappled creature(s)

Cast Leveled Spell (Costs 3 Actions). Cast one spell between 4th and 6th level. The spell chosen cannot be casted at a higher level.

Command Tentacle (Costs 3 Actions). Use Command the Spawn Tentacles or Rearrange Tentacle.

If Altellax's mythic trait is active, he can use the options below as a legendary actions.

Tentacle Eruption (Costs 4 Actions). All creature(s) within 20 ft. of a Spawn Tentacle(s) must make a DC 22 Dexterity Saving throw, on success half the damage (If a creature is Grappled by a Spawn Tentacle, their roll with disadvantage). Deal Force Damage equal to the that Spawn Tentacle's current hit points. Then dismiss all Spawn Tentacle(s).

Cast Greater Level Spell (Costs 4 Actions). Cast one spell between 7th and 9th level. the spell chosen cannot be casted at a higher level.

 Void Weave (Costs 5 Actions/ Recharge 6). Altellax absorb the void back into himself to empower himself. Within 150 ft. of Altellax remove all Void Leak, then he will regain Spell Slots depending on how many Void Leak was removed. A removed Void Leak is 1 Spell Slot, and so forth. (Example: 3 Void Leaks = (1) 3rd level spell, or (3) 1st level spells.) Spell Slots gains cannot go over Altellax's maximum of Spell Slots in that spell level.