Necromancers can use their own soul energy to condense into a so-called Soulseed, and then place that Soulseed into someone else’s soul. That person will then be under the complete control of the Soulseed’s creator. At the same time, between servant and master, there will be a spiritual link and ability to communicate.
Once a person is controlled by a Soulseed, deep in their mind, they will come to treat the wishes of their master as paramount. Even if one was ordered to commit suicide or commit patricide or matricide, it would be done without hesitation.
The only way for a Soulseed to be removed against the will of its creator, is to kill the creator of that Soulseed.