These are the magics I would consider. In no particular order. Let me know what you guys think
- Death curse
If you love someone, the curse simply kills that person. It also makes you immortal. It's an eternal condemnation to solitude. Can you imagine living, without ever being able to express feelings such as love, friendship, care etc? Poor Zeref couldn't even befriend animals
This is, IMO, the scariest magic in the series.
- Kyouka's senses curse
It can take away someone's senses or amplify them. What Kyouka did to Erza, towards the end of the fight, is one of the worst things that can be done to a human. If it weren't for the ultimate author wank of "she's Erza", she would've had one of the most painful death probably in all anime I have seen. Dying blind, deaf, dumb, unable to even smell, without senses of any limbs; all you feel is unspeakable pain.....and then you die.
- Nightmare
Midnight can completely warp the reality of the victim, and pretty much make the victim feel and experience anything. Including probably what Kyouka can do. Inflict unlimited torment until the victim's brain had enough and completely gives in. In real life, this can lead to permanent brain damage, PTSD and literal retardation. Maybe even death eventually, after a short life of torment.
- White out
Like you explained, it's not makes the victim pretty much a servant of the user, but it's also like a zombie apocalypse because it can spread. And the user (White Witch) can even lead to the death of the victim by making his MP zero, since mages can't live for long without their magic, due to magic deficiency.
- Mass manipulation (organ burst part of it)
Can distort the size of the victim's internal organs and bones, leading to a very gruesome death.
-Self Destruction Circle
Once cast upon the victim, unless they know the code, it slowly turns the victim's magic and power against it. While the victim is watching helpless, waiting for the inevitable - when it finally culminates, leading you to commit suicide by exploding from within.
- Memento Mori
Depends on whether you believe in concept of soul, reincarnation, after-life etc. If you do, this can be pretty scary. It completely erases the concept of the victim itself and reduce them to nothing. You're neither living nor dead, you simply do not exist.
- Ajeel's sand magic
Slowly kills the victim in pain, while sucking out every ounce of water from the body. All that will be left by the time Ajeel is done with you, will be your mummified body
There are more lethal and OP magics I've left out, because I feel like they just grant you a quick death, without being overly gruesome, suffering and/or terrifying.
He probably meant the DS magic of the arcane dragon - Acnologia.
