you were bitten by a werewolf and have been infected by it's curse. though thought of as a curse, there are many upsides to being a lycan as well

Keen Senses.   You have advantage on Wisdom (Perception) checks that rely on hearing or smell.

Improved Physicality.  your strength score increases by 1 to a maximum of 20

Shapechanger. You count as a shapechanger for spells(such as polymorph). As a bonus action, you transform into a special hybrid form for up to 1 hour. You can speak, use equipment, and wear armor while in this form, and can revert to your normal form as a bonus action. You automatically revert to your normal form if you fall unconscious or die. while in this form you have the following benefits:

• Feral Might. You have advantage on Strength checks and Strength saving throws, and you have a +1 bonus to melee damage rolls. This bonus increases to +2 at 11th level and to +3 at 18th level.
• Resilient Hide. You have resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks not made with silvered weapons. Additionally, while you are not wearing heavy armor, you have a +1 bonus to AC.

• Claws. Your clawed paws are a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal slashing damage equal to 2d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

• Bite. Your fanged maw is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d8 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike, and the target must make a constitution saving throw or be cursed with Werewolf Lycanthropy, your DC for this uses your constitution.

the curse of the Lycan also causes a bloodlust while transformed. If you start your turn with fewer hit points than half your hit point maximum, you must succeed on a DC 13 Wisdom saving throw or move directly toward the nearest creature and use the Attack action against that creature. If you’re concentrating on a spell or are under an effect that prevents you from concentrating (such as the barbarian’s Rage feature), you automatically fail this saving throw.

If you have your Extra Attack feature, you can choose whether to use it for this frenzied attack. If more than one creature is equally near to you, roll randomly to determine your target. Once your attack is resolved, you regain control of yourself.

During the full moon you are unable to control your self, you are forced into your hybrid form for the entire night, with the only desire being to kill everything you can sink your teeth into.