A long time ago, when the tribes were destroyed by the first Tamer, Shadow, A curse was layed onto the land. Plant life quickly covored everything destroying trees, camps, dens, rives, and the ocean. The shape of the lands dramaticly changed until it was completely different, but along with this was the Curse. Everything that died it's soul was destroyed instantly taking it from existance as if they never existed, This is why DarkForest and StarClan became empty. Though only 1 cat remained, a keeper, Silvermouth a young MedicineCat who worked in the acient times wasn't destroyed and was made leader of StarClan and Keeper of souls helping the living from dieing hoping the curse would break so they could rest in peace and hunt in starclan. But along with Silvermouth Shadow wasn't destroyed Either but isn't considered StarClan nor DarkForest, he's considered just a tamer and lives in the realm of the Dragons. The curse continues on and every cat (etc.) that dies is destroyed from existance leaving no wise cats to tell the future except for Silvermouth.
Though there are others besides Shadow and Silvermouth, they are known as the demons of the past souls, but they are simply fake and illussions meant to scare the living into believing the are real. But besides them being fake they can kill and take shape of living cats, some can only shape shift into 1 living cat while a rare few others can turn into anything. They are led by the illusion demon LionStones, the one who ruined the tribes completely causing all this destruction.
The undead souls never age and stay the age they died as. Almost all being young because they had died early
There is no way to break the curse from the lands so far and there wont be for a long time, hopefully no cats will die before then.
Curse: Soul destruction, all who die are destroyed from existance until the curse is destroyed
Curse cure: unknown