Author Existence Failure can be a sad thing. When a famous author dies and they leave only partly-completed works, never to be finished or seen by the general public.Or will they?
APosthumous Collaboration is when an unfinished work is completed by someone else for the sake of thepublisher's bank account fans.
The new author may have been close to the original one, or even helped with the work while the original author was alive, or just working off some discovered notes on what the original author had planned, but what's important is that the work has to have already been started by the original author before their passing; it doesn't count if someone just creates a brand new work using the dead author's characters or universe.
See alsoLiterary Mash-Ups.