Koškléftis

Koškléftis (魔法窃盗 (コスクレフタス), Kosukurefutasu lit. Ability Theft) is a rare Caster Magic that allows its user to gain the knowledge of the people they kill. Its current, most prevalent user is Xavier Tolva, a fairly new Dark Mage, but nonetheless, one with a significant bounty on his head.

Description
Koškléftis is a Caster Magic, though it gives the user no exceptional casting abilities. One of the most useless magics in terms of combat, it can only help its user after they've killed another person. As a passive ability, rather than needing to be activated, one must only possess Koškléftis to be able to use it. However, many users have noted that they feel more like a vessel to the magic, rather than the mage casting it.

When a user has Koškléftis, it allows them to gain the knowledge that another person possesses after they kill them. The way the target is killed doesn't matter, as long as the user is the last one to strike before their life ends. The knowledge gained includes physical capabilities, such as hand-to-hand combat and weapon wielding, as well as more common things, such as cooking and knitting. It does not pass on abilities, though gives its user the ability to pursue such things with vast knowledge, as well as giving them numerous counters for abilities they themselves don't use. The more people they kill, the more knowledge they gain, which in itself, becomes a burden after reaching a certain point. Considered a curse for some and a blessing for others, most people can't deny the absolute allure of this magic, regardless of the gruesome details attached to it.

A side effect of the magic is the intense, insatiable bloodlust its users have. They are considered to be highly dangerous people, even if they were once reliable and trusted members of society. Their behavior only worsens the longer they go without killing another human being, and some have gone mad trying to resist its pull. When faced with an opportunity, all thoughts seem to vacate their mind, totally focusing on killing. Their merciless, inhuman behavior has them labeled as demons, and some have questioned whether the magic truly is a Curse invading a human until they're left for dead.

However, once this bloodlust is satiated, users are gifted with a euphoria unlike any other. None have been able to articulate their exact feeling, some claiming it is close to their idea of heaven and others mentioning an "on-top-of-the-world" sensation. Regardless, it is an entirely positive and energizing feeling that encourages users to chase their high until their life ends. Once they're hooked on this, any other addictive behaviors seem to take the backburner, if not entirely disappearing, making way for Koškléftis' influence.

Abilities
The magic has no active casting ability and offers no guarantee that its users will achieve anything. Users must put in their own work to be able to use the abilities they gain knowledge of, and it's not uncommon for them to have trouble with a newfound ability, as it usually doesn't fit their body type. That being said, they are far ahead of the learning curve, as they already possess a wide pool of knowledge on the subject, rather than having to start at square one. Most mages who use Koškléftis are considered to be wildcards, unable to be pinned down as specialists of any kind, as they incorporate many different fighting styles at once.

Trivia

 * As I intended this to be rare, I won't be opening it up for free use
 * Credits for the name go to Astrarchē