You've made a pact with an other worldly creature or spawn. Much like a warlock it Grant's upon you abilities you could not normally achieve as a regular creature, at the moment with seemingly nothing in return. Your actions are your own but as you delve further into the pact your blood lust grows 

Signs of the pact

At 3rd level your pact is showing through your physical form, as part of a bonus action/short rest you can summon or disband them. Your hands take the shape of large claws emanating an elemental presence, you can slash or stab and use strength or dex. You lose the ability to equip non proficient weapons, and those you do have proficiency in do not gain proficiency bonuses. Your claws deal 2d4 slashing/piercing + dex/strength. The elemental presence forming on your new hands deal an additional 1d4 damage, and you can choose from Fire, poison,ice, or electric. At the same level your movement speed is increased by 10, and you may add half your dex proficiency bonus to your AC when in light armor or unarmored

Piercing lunge

At 7th level you can take an action to lunge and grapple onto any opponent within 15ft. Roll a grapple check to succeed in the lunge, you then deal 2d8 piercing + dex + 1d4 of previously chosen damage type. If you fail the grapple check you can roll to attempt a slash dealing 1d8 + dex + 1d4 slashing damage. Successfully landing the lunge leaves them grappled till your next turn and fail lunges leave you at disadvantage on their first attack against you until their next turn.

Growing hunger

At 10th level your normal attacks now deal 2d6 + dex + 1d4 and your piercing lunge can add an additional 1d4 damage on successful grapples. The pact has begun to show its downsides as a bloodlust begins to swell within you. You can choose to embrace or ignore its presence within you. Embracing it gives advantage on attacks towards creatures previously hit by you but they gain the same advantage towards you. You can only have advantage on one target at a time. Ignoring the bloodlust gives you +2 in constitution due to your resolve against the hunger.

Elemental shift

At 15th level you have grown attuned to your powers, and have gained the ability to shift which elemental damage your claws inflict or remove it, once per short rest

Embrace the pact

At 18th level as a bonus action you can engulf yourself in the elemental damage dealt by your claws. Doing so makes you immune to the same element for the duration. Your basic strikes deal an additional d6 damage and your lunge deals an additional d4. The affect last 1 minute or until your are killed. If you are killed before activating Embrace the pact you can use it to bring yourself back up to 1 hit point