Gain a +2 to constitution and a +1 to a score of your choice.

You don't need your head around to survive, but it does carry most of your senses

From your head emanates

Superior Darkvision

Telepathic radius of 10ft, you may speak to one or more creatures within that range regardless of shared language, but doing so requires your body to be inactive due to you breaking off connection with it to converse with the other target

When not within 10ft of  your head you gain disadvantage on all checks pertaining to sight and sound, unless you gain blindsight

Your head can be either held in one hand by you or another creature, or strapped to the neck by some device.

Due to the nature of your soul inhabiting a suit of armor wearing foreign armors do nothing for you, but you do have a natural AC of 15+ Dex modifier due to your unnatural flexibility being one with the armor. You do gain the benefits of a shield when equipped. You are considered wearing medium armor for all purposes of feats, stealth, and unarmored defenses.

As you have had a brief brush with hell you know how to speak, read, and write Common and Infernal