The darkness consuming consumes you limb by limb. Choose a limb that is consumed by the darkness. Arm: your right/left arm (whatever you chose) is a purplish if not black color and your fingernails are now claws and you gain a natural weapon 1d6 damage, Leg: Your right/left leg is a purplish if not black color and you have a speed of 45ft, Eye: your right/left eye is purple and has a black pupil causing you to gain 120 ft darkvision in that eye.

Since you aren't a full vampire you still age the same as you were before the incident.

Demonic Vampires are being consumed by darkness which clouds their judgment and tempts them to evil but, sometimes the mentally strong can resist the temptation.

Your size is what size you were when you were bitten

Thanks to your vampire blood, you have superior vision in dark and dim conditions. You can see in dim light within 90 feet of you as if it were bright light, and you can see in darkness within 60 feet of you as if it were dim light and you can't discern color in darkness.

You have immunity to charms and sleeping spells

Your constitution and dexterity scores all increase by 1.