You finally got a girlfriend, but it seems like she was a Devil and nearly killed you, you almost died until Rias saves you, you now live for her sake as she is also a Devil. She now lives in your house and your parents are fine with it for some reason. You head to school with Rias and she decides to take you to her club

Rias: Welcome to the Occult Research club. You will now be one of my pawns, so serve me well!

she smiles and you see all the other club members who are also devils