Located in the depths of the Demon's Ruins in the Lost Izalith, the Bed of Chaos is a multi-stage boss fight that will not reset. To get to the Lost Izalith you must go through Blighttown, Quelaag's Domain, Demon Ruins, into the Lost Izalith. You will require the Orange Charred Ring in order to survive down here because you must take long walks on lava. Without the ring you will die. You can obtain this ring from the Centipede Demon just before the Lost Izalith area.
The Bed of Chaos is a 3 stage fight. When you first encounter it, you will notice glowing spots on either side and the center of him. Dodge his attacks and get into the glowing area and look for a small demon. Once struck it will move to the next stage and grow one large fire arm. Repeat this for the other side and another large arm will appear. The center is the hard part. The floor will start crumbling only leaving you specific areas to move on including right in front of it. But if you can spot it you need to jump straight at him to land below on a small pathway. Follow this up dodging his attacks to the end and strike the last demon to defeat it. The timing of the attacks at stage 3 is the key to your success.