At level 2 the Bone Demon unleashes some of its hidden magic and can choose one embedded attributed spell from their chosen element
Fire: Fire Bolt
Ice: Ray of Frost
Poison: Poison Spray
Necrotic: Chill Touch

The spell can only be used once as a bonus action but after performing the spell it cannot use it again until a long rest is finished. It increases the cantrip's damage at 5, 10, 15, and 20 by adding a 1 extra dice. 

The bone demon has boney wings that sprout out at level 10, and they can magically enchant the wings to give themselves a flight speed of 30 feet for an hour. They can only use this ability once per day.

At level 5 it cannot be healed normally but instead leaches the life forces on souls. It will have to touch a living or undead creature to heal itself. It can heal itself to 1d5 times its level.

At level 20 the Bone Devil has fully transformed and can fly at any point in time. Its face is a horned skull and feeds on souls. 1 soul can feed it for a day. Your tail is now skeletal, with protruded spines and tightly stretched skin and is a natural weapon, which you can use to make unarmed strikes. If you hit with them, you deal piercing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

Your constitution and charisma score increases by 1

Tieflings mature at the same rate as humans but live a few years longer.

Bone Devils are very corrupted Teiflings, and will eventually transform. They are evil.

Tieflings are about the same size and build as humans. Your size is Medium.

Your base walking speed is 30 feet.

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

You have resistance to fire damage.

You can speak, read, and write Common and Infernal.