You gain access to the Insufferable Crypt via the Ancient Sewers. To gain access to the ancient sewers you'll need to have the ram rune and so forth.
Ancient Sewers are located in the Toxic Sewers. Somewhere inside the location will be a breakable floor which you use the ram rune to break through and then work your way to the Ancient Sewer Floors till you get to the "Insufferable Crypt" door.
This room is just a single boss room where you face the boss Conjunctivius. Upon beating the boss you'll also get:
I've linked a video below as well as a full playlist showing you how to get all areas/runes.