What Happens Inside the Dungeon - Vol. 1 Ch. 10

[MangaDex]

 

[Gravitypull]

I'm guessing that tattoo either came from the dragon or when she used all of her power when fighting said dragon

 

[tyrpios]

So she will become a succubus?
This also makes me think about the cloaked guy she met at the beginning and spread some dust on her clothes

 

[Deathhappens]

Her sister stole her boyfriend, didn't she.