Limbo is not a place, but the absence of a place. It is "where" Heartless Celestials go when vessel-killed, with the exception of Kyriotates and Shedim, who are merely forced in celestial form, generally taking dissonance in the process. There they wait, slowly accumulating essence in quantities far greater than they could under normal circumstances, until they are able to use it, as Ethereals do, to form a new vessel. Of course, spending months or years in complete sensory deprivation tends to drive those with low wills a bit nuts, and as a result, they may lose the essence they've accumulated and have to start over again. Since there is a minimum amount of essence needed to create even a lousy vessel, this can become a vicious cycle which leaves the increasingly insane (often Discordantly so) celestial stuck in Limbo indefinitely. Friends on the outside can help by casting essence into the void "with their friend's name on it." If the intended recipient is in Limbo, he will receive the essence, and also gain a small bonus on his next will roll to avoid losing his accumulated essence. If he is not, the essence is simply lost. The sender has no way of knowing, unless and until his friend emerges from Limbo.