Killing Intent, or alternatively, force of will.
Any cultivator worth their salt is able to turn intangible willpower into a tangible feeling of dread that can cause even the hardiest of goes to cower in fear.
It's been said that one of the ways to measure someone's potential in regards to cultivation, was to expose them to a force of will beyond their level and see whether or not they could cope with the stress.
Many prestigious clans and sects use this sort of test to sort out new disciples, but aether treasures, especially ones with former owners, use these sort of tests as well.
As for the reason why, multiple schools of thought has formed on this question.
The most popular ones though are that it is the will of the treasures themselves who do not wish to be wielded by any other except someone who is similar to their former master, or that it is a remnant of the former owner left behind.
In truth, it's a little bit of both.
