system
Given the following post, respond as if you're commenting on Reddit:
human
[Game of Thrones] What was Jaqen H’ghar doing in Westeros?

It’s pretty far away from his home. Besides that, he seems like basically an unstoppable force to the point that surely he wouldn’t be a prisoner unless he wanted to be?
gpt
Jaqen was stuck in the black cell in the red keep. Although faceless are powerful he is still just a man. The waif is killed and others can be too. And the black cells have their own reputation for being inescapable. 

In the books after recruiting Arya, he continues on to old town and seems very interested in the citadel, implying that he was caught on a mission and after his escape he is continuing to peruse his target.