Death Eaters can use these spells, but remember not all dark wizards are Death Eaters. Basically, anyone who is evil enough to use these spells can use them. You might argue that Harry used Imperio and Crucio in DH, but that would just lead to needless argument. You can use dark spells if you believe that your characters personality would suit it. Of course, someone like Bellatrix etc. will be able to use this form of magic better. As she explained in OoTP, you have to mean it. (read laws of magic for more information)
There are several versions of dark magic.
THE UNFORGIVABLE CURSES
Often considered to be the most dark spells.
Avada Kedavra / The killing curse
Avada Kedavra is a spell that instantly kills whoever it is cast upon. The spell cannot be blocked by any spell.
Crucio / The Cruciatus Curse
This spell makes the victim experience unbearable pain. As you can see when it is used on Harry in GoF, he could barely move after it was used on him - an example of its power.
Imperio
This spell gives the caster complete control over the victim for some time. In fact, the caster can gain permanent control over the victim. However, the victim can become free of the spell through strong willpower.
NECROMANCY
A wizard who has mastered the arts of dark magic can take control of many corpses and make them do his bidding. These are called inferi. To use necromancy, you must be a graduate death eater.
OTHER DARK SPELLS
Morsmordre
The spell that summons Voldemorts dark mark into the sky.
FiendFyre
Only NEWT level students and above can use this. Fiendfyre creates a dangerous and uncontrollable fire which spreads continuosly and takes the form of various creatures.
Gladius
A slashing motion which sends a streak of purple flames. If hits, it causes serious injury.
Cruor Sacrificum
Advanced level. An enchantment which blocks all people from entering an area unless they pay blood sacrifice. (DH the cave)
Cruor Atrum
An enchantment which blocks all people from entering an area unless they have the dark mark.
Morsomnium
A curse that makes a victim fall asleep and suffer nightmares for as long as the caster makes him, unless they can throw the spell off. (like Imperio)
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