Special Trait Goddess of Life. Xerneas is the Arcean goddess of life, rendering her as one of the most powerful Arcean beings. Because of her domain, necrotic damage is useless against her. Any healing effects (such as from cast spells or enchantments on items) are twice as effective on her or her allies.

Special Trait Gift of Life. While in battle, if Xerneas and her allies cannot suffer instant death. If they need to roll death saving throws, they only need to roll a 5+ to succeed.

Spellcasting. Xerneas cannot cast spells.

Action Healing Aura. Xerneas emits a powerful aura, and can use this to heal up to 3 targeted allies of her choice (she can choose to heal herself) and heals them by 2d4 + 2.

Action Melee Attack. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 2 (1d4 + 6) radiant damage. 

Action Ranged Attack. Ranged Weapon Attack: +9 to hit, range 40/40 ft., 5 target. Hit: 1 (2d10 + 4) psychic damage.

Xerneas doesn't have bonus actions unless an ally does something to give her bonus actions.

Reaction Healing Aura. Xerneas emits a powerful aura, and can use this to heal up to 3 targeted allies of her choice (she can choose to heal herself) and heals them by 2d4 + 2.

Xerneas can use her Legendary Actions any time during her turn.

Legendary Action Geomancy (Costs 2 Actions). Xerneas heals her and every one of her allies by 10 hit points; her and her allies' AC and Speed increase by 5; and all psychic and radiant damage dealt by Xerneas and her allies is doubled. This action is charged in the first turn Xerneas uses it. and then used in her next turn. These effects go away on Xerneas' third turn after Geomancy is used (5 turns after Xerneas starts Geomancy). While Xerneas is using Geomancy, she cannot move or use any actions with the exception of communication to others.

Xerneas is not mythical, as she is more than simply confirmed to exist.