The Crimson Halls of Insanity were the deepest level of the prison that sealed Yogg-Saron away. More precisely, they were part of the prison.
Ulduar's interior was an immense complex composed of layer upon layer of different planes. The Titans had designed the prison this way to prevent Yogg-Saron from escaping. Even if the Old God broke out of its cell, it would still have to break through numerous planes, one after another. That would take an enormous amount of time, giving Ulduar's Titan Keepers time to respond.
At Ulduar's height, most of the world's Guardians had been stationed here. As long as nothing went wrong with its defenses, Yogg-Saron could forget about escaping for the rest of its life.
But however comprehensive a prison's defenses were, they still needed jailers to maintain them. Ulduar's greatest problem was that its jailers had broken down before its defensive system did.
The Crimson Halls of Insanity were a layer of space near the innermost part of Yogg-Saron's cell, but at some unknown point, they had been completely abandoned. The Titan Keepers had given up this place and instead built an extraordinarily sturdy Wall of the Ancient Guardians outside the Crimson Halls of Insanity, trapping the darkness leaking from within behind it. They had believed this would halt Yogg-Saron's steady encroachment upon reality, but they had failed.
Yogg-Saron might not have been a match for the combined strength of these Guardians, but there was no doubt that the Old God's power was stranger than they had imagined. The wall had halted its advance, but it could not block its thoughts, which seeped through every crack. And so, amid the madness sweeping over everything, the touch of chaos came surging in, like black tentacles wrapped around Ulduar's body, constantly, frantically draining every last drop