This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / W.I.T.C.H.
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 existence, 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 they're not sentient. They come into existence 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...
Similarly, think of how Elyon was trapped alone in a jewel for the majority of season 2—from the beginning of summer (i.e. June) to the middle of December, provided they're following the typical American school year. Of course, she picked up some company in R is for Relentless, but that still means she was stuck there alone for around four months. It's amazing she didn't go stir crazy!
Matt's first transformation into Shagon.
Similarly, Mr. Huggles first transformation into Kor.
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.
Cedric. Imagine a Consummate Liar so good with words and cunning that he can turn someone against their best friend with a single talk backed by a bit of carefully selected and presented truth, give him the ability to change his looks at willanddisguise himself through non-magical means (redundant? No, it allows him to bring with him an ogre that can't disguise themselves with magic), and imagine what it would be to have him as an enemy…
Almost forgot: those above are not his only skills. He can fire energy blasts and Eye Beams, cast various spells (and is competent enough that he could teach Elyon to use hers and make her more than a match for all the Guardians in a couple weeks), and when he's after you he can track you down everywhere and anywhere, as shown when he hunted down Elias Van Dahl in the seventeenth century.
With the description above, you could think that locking him in a form and knowing exactly who he is and that he's an enemy is enough to stop him. The Guardians thought so, and so did Matt. Then, in the Book of Elements arc, Cedric disabused them of this notion by tricking Matt into trying to open the Book, and getting sucked into it, by shouting at him to not come close to it when Matt saw the book embedded in a wall and got curious and then leaving the room with a perfectly believable excuse… And he was improvising. Now you see why Orube's reaction at first seeing him on Earth was fear, don't you?