But stalking an undead is not always easy since don't they have something like life sense? Where they know someone is in their area?
One of a certain villagers tasks was to actually assassinate ahtieli (or satebos- maybe it was either lich at the time) so I am pretty confident it is possible, but highly improbably you'll ever get enough stalks for the try to work. Undeads almost always have the see hidden mask or emperor powers. Cleave away, might get lucky.

A clever assassin maybe could pull it off by finding ways to avoid detection, but again, undeads rarely get to that level by putting themselves in positions where there is risk of being assassinated, they tend towards the highly conservative.