I've decided to test myself by making a level 1 vestige. They're the hardest to make, balance-wise.
The Void Before the Altar
1st Level vestige
Sign: You grow the curling horns of a ram.
Manifestation: Amon manifests in a burst of black smoke, howling foul curses at his summoner. He possesses a black wolf’s body with a ram’s head and a serpent for a tail. His mouth is ﬁlled with sharp teeth, and ﬁre escapes it when he speaks.
Darkvision: You have darkvision while bound to Amon.
Fire Breath: You can vomit forth a line of fire as a dragonborn. Once you do so you must take a long rest to do so again.
Ram Attack: The horns you gain as Amon's sign function as a natural weapon that deals 1d6 bludgeoning damage on a hit. [(I forgot if you can charge in 5e, so ignore this part if you can't) It deals an additional 1d8 damage if you charge with it.]
You have advantage on all ability check and attack roll contests made against other binder. When you reach 13th level, the benefits of this ability also apply to contest rolls made against Aberrations, Celestials, Fey, Fiends or undead, as well as mortals possessed by these creatures or have formed a pact with them. [Thanks to Zahn for this ability!]
Scholars claim that Amon is what remains of the personality of a god who died of neglect millennia ago. Once worshiped by thousands, Amon eventually lost his faithful to more responsive deities. His will was strong enough, though, to resist eternal sleep on the Astral Plane. Since his demise, his half-existence as a vestige seems to have dramatically changed his appearance and personality. Once a calm and wise protector, a god of light and law, Amon is now a foultempered and hateful spirit.