There's also at least one magic site which can horror mark units or commanders in the same province.
There are a number of magic items which can horror mark the user, but that is -normally- avoidable, assuming nobody has enough time and resources to prepare an assassin for horror marking and then casting Return.
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Are we insane yet? Are we insane yet? Aiiieeeeee...
There are some problems with horrormark that will be adressed in the patch. You will not get rid of the marks, but they will be less likely to attract horrors and the doom horrors in particular. Other changes might also appear.