Celestial Familiar

Spellbound. While within 100 feet of its summoner, the Celestial Familiar has resistance to all damage, as if benefitting from Warding Bond, and is able to telepathically communicate with its summoner. The Celestial Familiar also has disadvantage against Banishment, Banishing Smite, and Dispel Evil and Good, and other spells and magical effects that banish extraplanar creatures. Additionally, if the Celestial Familiar dies, its summoner must succeed a DC 20 Constitution Saving Throw or suffer 10d10 Psychic damage and 3 levels of exhaustion or half damage and only 1 level of exhaustion on a success. This applies regardless of distance, including being on another plane of existence.

Shapechanger. The Celestial Familiar is able to polymorph into a Large, or smaller, Celestial or Beast less than or equal to its CR, gaining the Strength, Dexterity, and Constitution scores of its new form, but maintaining its Intelligence, Wisdom, and Charisma scores. It also gains temporary hit points equal to its new form and its AC but loses the Shared Spellcasting trait while in this form.

Shared Spellcasting. The Celestial Familiar can cast one spell from the prepared spells of its summoner at 5th level using the summoner's spell save DC and spell attack bonus. This ability expends one of the 5th level spell slots of the summoner, but is treated as a separate spell when making concentration checks, or casting another concentration spell. In addition, the Celestial Familiar can cast the following Cantrips:
Cantrips (at will): Spare the Dying, Sacred Flame

Polymorph. As an action, the Celestial Familiar polymorphs into a Large, or smaller, Celestial or Beast less than or equal to its CR, gaining the new forms AC, HP, STR, DEX, and CON. While in this form, the Celestial Familiar cannot benefit from its Shared Spellcasting trait.

Shared Spellcasting. Melee/Ranged Spell Attack: Spell attack chosen from the summoner's prepared spell list, using the summoner's spell save DC and spell attack bonus.

Sacred Flame. Ranged Spell Attack: DC 15 DEX save, range 60 ft., 1 target. Hit: 9 (2d8) Radiant damage.