In Eldar mythology it is also considered to be the realm of the Laughing God , a land where everything is nothing and nothing is everything.
Sometimes, when someone who challenged Fate in their life dies, they go to Limbo, a place of choice, between darkness and light. Limbo is never the same for two different people and the choice they make will always be different, it might be eternal life in exchange for servitude or eternal damnation in exchange for their dreams coming true. This is a place beyond the boundaries of material universe, but it's not part of the Warp, it's between those two.
Chapter Master spent 4 real-space years (317-321.M41) inside this Emperor-forsaken place (though to him it felt like mere hours or days), and was presented with the choice, join his already fallen brothers, or fall even further than they, and succumb to the whims of chaos. A lesser man would have undoubtedly chosen the second life, and indeed, some parts of Chapter Masters mind wanted to go to chaos, for all the temptations it promised, the salvation of his Chapter, eternal strength, the love of an Eldar.....
But Chapter Master was not a lesser man, he refused to bow to the whims of fate, with a mighty roar, he shattered the boundaries of Limbo, and struck a deal with the Laughing God himself, last from the pantheon of the Eldar, heretical deal perhaps, but the only viable option, for to have accepted his ultimate rest would have been to condemn his Chapter to annihilation.