You were struck by a wave of magical energy that imbued you with powers of your own. This surge would have killed most but for some unknown reason you absorbed this power and have learnt to harness it. Whether it was a miscast fireball or a dying dragons breath, your physicality has been changed and is prone to more. 

Adaptable Form

You have the innate power to absorb elemental damage after a period of exposure. Whenever you take acid, cold, fire, lightning, or poison damage you can choose to allow it to change you. You gain resistance to the triggering type of damage for 1 minute after taking one of these damage types (not reducing the damage of the initial hit). This feature can only make you resistant to one damage type at a time. Feel free to expand upon how your physicality changes when you use this feature. For example, absorbing fire damage may turn your hair to flames for the duration and acid could harden your skin into a hard chitin. 

Elemental Dispersion

While your Adaptable Form ability is active you can add your charisma modifier to the damage of any spell you cast that deals the triggering damage of your Adaptable Form or to any melee weapon attack. The damage type is the type that triggered your Adaptable Form.

Preemptive Adaptation

Whenever a spell you cast would cause you to deal acid, cold, fire, lightning, or poison damage to yourself, you can choose to have that spell activate your Adaptable Form ability and you take no damage from it or any other spell you cast that deals that damage type until your adaptable form drops.

Controlled Absorption

You learn the spells "Dispel Magic" and "Counter-spell". They do not count against your spells known and you can cast them both once per long rest without expending a spell slot. Feel free to describe how using this ability to absorb magic changes you. For example, Dispelling the illusion of a cave wall covering a passageway could give your skin a stony texture whereas counterspelling a "Charm Person" spell could leave you felling light headed.

Evolved Adaptation

While in your adaptable form you add +2 to your AC