You are correct, it is Hathsin, which is technically two prisons, or a twofold prison, as Leras used it to imprison a sliver of Ruin's power, and Rashek used it as a prison/torture camp for Skaa criminals.
Oooooooh, good guess, but no, it is not Rathalas.
1. A major death or deaths happened here that drastically shifted the course of a protagonist's life
2. A Shard Vessel has been here
3. This place no longer exists
4. This place contains at least one prison
It is not the seat of Rashek's power.
1. A major death or deaths happened here that drastically shifted the course of a protagonist's life
2. A Shard Vessel has been here
3. This place no longer exists