Thinking about this Circle more, I decided to add a few more elder spirits to provide analogs of sort to the divine domains.
Spoiler: Additional Elder Spirits
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Elder Spirit
Devourer. As long as your spirit companion is summoned, you have advantage on Charisma (Intimidation) checks. Additionally, each creature with 30 feet of your spirit companion that can see them has disadvantage on saving throws against being frightened.
Moon Spirit. As long as your spirit companion is summoned, you can cast the disguise self spell at will.
Watcher. As long as your spirit companion is summoned, you gain the benefit of the comprehend languages spell.
Avatar of the Old Faith
Devourer. Your spirit companion’s weapon attacks deal an additional 1d6 necrotic damage. Additionally, your spirit companion regains hit points equal to the necrotic damage dealt by its weapon attacks.
Moon Spirit. Whenever you summon your spirit companion, you each gain the benefit of the invisibility spell for as long as it’s summoned. You do not need to maintain concentration on this spell, but it still ends like normal for you or your spirit companion if either of you attack or cast a spell.
Watcher. Your spirit companion gains truesight out to 60 feet, and you can perceive through its senses as an action. While doing so, you are blind and deafened, and can stop perceiving through your companion at any time.
Any criticisms of the second version of the class would be appreciated!