Special Trait God of Order, Balance, and the Ecosystem. Zygarde is the Arcean god of order and balance and the ecosystem, rendering him as one of the most powerful Arcean beings. Because of his domain, all necrotic damage is halved (after any other effects on the damage) when dealt him or his allies. All extra damage dealt because of things like critical hits are nullified.

Special Trait Reign of Order. While in battle, if Zygarde reaches half of his total hit points, he is able to transform into his Complete Form when he uses Power Construct.

Special Trait Prevailing Balance. While in battle, if at least half of Zygarde's allies are unconscious or dead, Zygarde can use his powers over balance to revive them and get them back in the action, completely fine and at full hit points. He then gets 1 bonus action. 

Spellcasting. Zygarde cannot cast spells.

Action Call Cells/Cores. Zygarde calls all of the Cells and Cores within a 200 ft radius to him. Roll 5d10 for the number of Cells. Every 5 cells heals Zygarde by 1 hit point (rounded up). Roll 1d4 for the number of Cores. Every 2 Cores increase Zygarde's AC by 1 (rounded up). Zygarde then gets 2 bonus actions for the current turn. Zygarde cannot use this more than once per turn.

Action Aura Break. Zygarde breaks all aura-related effects from up until now in the current battle via a powerful aura of his own. He can choose to keep specific aura-related effects in play if he chooses to.

Action Melee Attack. Melee Weapon Attack: +6 to hit, reach 10 ft., 2 target. Hit: 1 (1d10 + 4) force damage. 

Action Ranged Attack. Ranged Weapon Attack: +3 to hit, range 60/60 ft., 10 target. Hit: 3 (1d4 + 1) piercing damage.

Zygarde gets bonus actions for the current turn after the effects of Prevailing Balance or after using Call Cells/Cores.

Reaction Power Construct. If Zygarde reaches half his total hit points, he can use this reaction to change into his Complete Form. In his Complete Form, his movement speed will decrease by 10; but his AC, total hit points, and all force and/or piercing damage dealt by him increase by 5. He also heals by 30d10 + 100. After changing into his Complete Form, he cannot change back until the battle ends.

Zygarde can use his Legendary Actions any time during his turn. 

Legendary Action Land's Wrath (Ranged Attack) (Costs 1 Actions). Zygarde summons the power of the ecosystem to rise up from the ground and hit all of his opponents. This attack goes through AC. +2 to hit, range 50/50 ft., X target. Hit: 2 (2d4 + 2) force damage.

Legendary Action Thousand Arrows (Ranged Attack) (Costs 1 Actions). Zygarde fires hundreds of Cells into the air. The Cells then shoot downwards at all of his opponents (the Cells then return to Zygarde immediately). Any hit targets that were flying or levitating are struck down to the ground and are prone. This attack goes through AC and always hits unless a target is underground. +2 to hit, range 60/60 ft., X target. Hit: 5 (1d4 + 2) piercing damage.

Legendary Action Thousand Waves (Ranged Attack) (Costs 1 Actions). Zygarde fires hundreds of Cells around him, as if they were a giant wave going in all directions. The Cells move around to end up in a chain-like formation. They then suddenly tighten and deal damage before returning to Zygarde. Any hit targets are restrained and cannot escape. This attack goes through AC and always hits unless a target is in the air. +2 to hit, range 50/50 ft., X target. Hit: 1 (2d4 + 2) force damage.

Legendary Action Core Enforcer (Ranged Attack) (Costs 1 Actions). Zygarde gathers his power and charges a huge beam of energy. He then fires the beam at a target, inflicting enormous amounts of damage. +2 to hit, range 70/70 ft., 1* target. Hit: 1 (3d10 + 10) force damage. *4 targets if Balance Area was used.

Legendary Action Balance Area (Costs 1 Actions). Zygarde fires thousands of cells to cover an area with a 250 ft radius from himself. The cells are sent to strategic positions instantly so Zygarde has legitimately no blind spots and can see everything in the area. Zygarde can then roll a 1d20 to dodge any attack, and does so successfully (no damage taken) if he rolls at least a 5. However, Zygarde cannot dodge attacks that never miss. Core Enforcer also can hit a total of 4 targets after using Balance Area.

Zygarde is not mythical, as he is more than simply confirmed to exist.