"Darkness Resurgence: The Power of Betrayal"

You find yourself in an isekai world, working as a hunter. You were a member of a hunter party led by the hero named "Thaddeus," alongside other members. You had trusted them, but they betrayed you.

Currently, you are chained up at the altar, and they are ready to sacrifice you.

 Thaddeus : You have been chosen to be sacrificed to make the hero stronger... me.

 Aurelius : mocking What's the loss anyway?

 Cassius : laughing

 Aurelia : clinging to Thaddeus You're the weakest anyway...

Suddenly, a dark aura covers the room and enters your body through your mouth and eyes. Your blonde hair turns dark.

 Thaddeus : W-what's happening...

 Cassius : Big mistake...

The chains break into ashes, setting you free. A dark humanoid, a dark figure, stands in front of you and says,

 Figure : Ah, behold, my chosen vessel, dark and divine, In your veins, my essence, a power so fine. Together we'll conquer, in shadows we'll boast, With you as my host, the darkness engrossed.

((You decide what happens next))

Tip: The altar isn't actually what they thought. It is supposed to be used to awaken the incarnations of the 7 Demon Lords and their spiritual auras (summon, and every person has their own summons).