With a little research, I found a book in the Library of the Island of the Kings, which tells a story related to Undead Jester.
Fools From Beyond
"Many of the dead leave some kind of unfinished business behind. Some of them manage to stay in the world of the living as some form of ghost or undead, but most of them simply move on to the afterlife. But even there, some of them simply don't give up. And as there is little in the world as persistent and diligent as a bad entertainer, many of them stay at the border between the two realms and contemplate their ill fates. They mourn the fame they never acquired and which they feel was unjustly stolen from them. They partner with similar minded individuals and complain to each other how cruel the world has treated them. Of course none of them is actually listening to each other because they are too absorbed with their own misery. So they grow louder and louder and over the course of time, their monologues are heard everywhere in the plain of death. Sadly, one of the things even more persistent and diligent as a bad entertainer is a whole bunch of them. In the end, they grow that loud and annoying that the guardians of the gates of death themselves lose their patience. So once a year they throw the annoying fools out and banish them to the realm of the living, knowing that they would not stay there for long given their annoying nature. So once a year hordes of undead jesters raid the cities where they once were seeking fame and glory. As undead, they try to terrorise the living, however, as they are even worse as undead than they are as entertainers they pose only a minor threat. Usually they find themselves back in the afterworld quite soon, and for another year, they dream of greatness that was never theirs."
The reason for the invasions means the search for something they never had when alive and torments the world of the living once a year, it was the only thing I found related.
Fools From Beyond (Book) - Tibia Wiki