What is Ascension?

Solution 1:

Passing through the mysterious astral gash after the main quest is completed will allow you to enter the afterlife but you cannot return to your old life so make sure you have no Unfinished Business.

Though there is some interesting content in the afterlife, there are no real quests or advancement or anything, unless you count the whole new adventure you get to start out on as a level 1 by ascending.

Ascension allows the player to "reset" their character to try out any of the other classes, each with their own content, as well as voluntary tasks or restrictions available only to players that have ascended. You will get to keep all of your non-quest items (though they will be unavailable for a certain amount of time) and some (likely one) of your skills.

Solution 2:

Once you've completed all the main quests, you are given the option to ascend. This ends the life of your character, moving on to the astral plane. At this point, you can choose a new incarnation of your character, essentially starting the game over, optionally accepting voluntary challenges.

You accumulate all the karma gained over the course of that character's life with which you can improve your character by allowing you to remember skills that you've learned in previous incarnations or granting exceptional food or equipment to take with you from the astral plane. The more challenges you imposed upon yourself in that incarnation, the more karma you earn once you've ascended.

Any equipment you've accumulated in previous incarnations is put into storage to which you have limited access over the course of your next incarnation. Depending on which challenges you've accepted, you can pull a small set of things out of storage each day for the first 1000 adventures (during which, you are considered in the ronin phase) or not at all until you've defeated the sorceress.