I have just taken Rabina from Hla Oad to the Argonian Mission house in Castle Ebonheart. After i got there and she said she was safe. So i went throu the door behind Im-Kilaya and followed the corridor and turned right. At the end of this corridor i found a looked room whit a slave inside, and she said that i need a key to free her so i am wondering where i can find the key.

Probably in some chest or dresser in the mission, maybe even in one of the Argonian's pockets.