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.
Prince Phobos is a sadistic, powerhungry, and vainglorious madman who only cares about acquiring magical power. He will crush anyone who stands in his way, even if they are his own family. He takes great glee in manipulating and fooling his sister Elyon with his Faux Affably Evil demeanour until her power has reached its peak. And when the charade is up, Phobos mocks his own sister's naivety and has no problem binding her to a throne and zapping her of her magic. The only thing scarier than his ruthlessness and lack of empathy, is his short temper, and his habit of severely punishing those who fail or betray him.
Nerissa is even worse than Phobos. A former Guardian of the Veil and original owner of the Heart of Kandracar, Nerissa's desire for power corrupted her, leading the Oracle to strip her of the Heart. In retaliation, Nerissa murders her best friend and fellow Guardian Cassady, and is imprisoned in a tomb. Years later, she escapes (the reason different in the comics and animated series) and vows revenge on everyone. She appears as a hideous, scarred old hag, just as manipulative and amoral as Phobos, and to hell with anyone who gets in her way. She is a horrifying master of the Batman Gambit and the Xanatos Gambit, manipulating others for years to ensure she gets what she wants.
Made worse in the animated series. Perhaps her most vile action was murdering the Mage and posing as her, before having sex with Julian to give birth to Caleb, wanting to have the perfect child to serve her during her plan for universal domination.
Being a Guardian can be extremely nightmarish. The girls have faced death multiple times, and Cassidy did die. Who knows how many Guardians died before her? Not to mention they don't even get a guidebook or anything on how their powers work, they just get thrown into battle and have to figure out everything by themselves. And for someone who's supposed to protect everyone in the universe, the Oracle can be downright evil. It's a very good thing Yan Lin took over, as she's much more sympathetic and understanding.