This is the first non-linear area you access. To be able to reach it, you'll need to have the Vine Rune which you obtain from an elite in Promenade of the Condemned.
Once you're undoubtedly back in the Prisoners Quarters, the access to Toxic Sewers will be accessed via this area. Located here is two vines, one is for a small secret area and the other is the direct passage to Toxic Sewers.
I've linked a video below as well as a full playlist showing you how to get all areas/runes.