Ah yes, it was revealed to me when I took the Harry Potter Sorting Hat Test that I would be a Slytherin. While that does not necessarily suggest evil, I must say that some of the spells and such are quite delightful. Here is a list of dark spells I would be willing to use.
Crucio (The Cruciatus Curse)- This spell would be most delightful in my opinion. If someone really got on my bad side, I would not hesitate to use it. Crucio is from the Latin word meaning "I torture". As such, the spell causes pain so unbearable that it can even cause insanity. Though I would likely not use it to such an extent, I would use it if someone really crossed me.
Note: This is an unforgivable curse, and I would only use it if I thought I could get away with it
Imperio (The Imperiatus Curse)- Well, of course, I would use this? What kind of pansy ass dark wizard or which doesn't want slaves to do his or her bidding? The spell is said to make a person's free will separate from his or her body. However, some wizards, including Harry Potter himself, can resist the curse.
Note: This is also an unforgivable curse
Adava Kadavra (The Killing Curse)- I dare say this was Voldemort's favorite of the three, and it is the only one (as far as the books are concerned) that Harry Potter never used, though it is likely he would have occasionally used it as an Auror as an adult as it was often said to be a last resort. This is probably the worst of the three, and I would only use it for one of two cases. 1) Someone had messed with my family or a close friend. 2) If I, or someone close to me, were in danger.
FiendFyre- This is one of the most dangerous spells. It is so powerful as to be able to destroy a horcrux, meaning that anything it touches cannot be repaired by magical means. It causes a huge plume of fire to shoot at it's intended victim. However, it is very advanced, and only a wizard highly skilled can use this spell properly. Thus, I would be wary of using this spell, as it has been known to actually kill the caster.
Sectumsempra- This spell is a nasty one indeed. It cuts the intended victim as if by an invisible sword. Without some magical healing, I suspect the victim will bleed to death. This was a spell created by none other than Severus Snape.
There are also spells and other dark magic I would dare not use under any circumstances.
Serpensortia- This spell causes a snake to appear. Ummm... eww.
Inferi- Well, dead stuff gives me the willies, and inferi are worse than zombies. 'Nuff said.
Horcrux- I'd rather keep my soul attached, thanks. Lord Voldemort killed 6 people to split his soul in 7 ways (including the small piece still left in his body). Also, Harry Potter was one of the horcruxes. Thus, there were 8 pieces of his soul all together. This is probably the darkest of all magic in the Harry Potter universe, and it is more evil than I'd be willing to do.
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And yes, I know that these things are impossible in real life, but a guy can dream, can't he?