The Sword of Exile. It doesn't just kill you, it erases your very existence, and everyone's memories of you disappear. Essentially, if it had retroactive erasure as well, it would literally wipe every trace of you out of existene, but thankfully, your deeds themselves are not erased.
The astral drops exist in some kind of limbo. There's nothing on this place except for the astral drop, who is alone, can experience feelings like anyone else and... she's trapped there unless the Guardian that created her takes her out of there.
The astral drops probably have it worse in the cartoon, even though their not sentient. They come into existance for a while, and after they're not needed they're made to go "poof". As if they never existed in the first place. Kinda sad really...
In "Z is for Zenith", W.I.T.C.H. were close to losing their humanity. They would have remained in their super powered state, if it weren't for their loved ones.