The first Enemy signature Weakness. Looking at the two elements, the effect and the discard condition, we get:
The effect: A relatively mild Enemy, the ghouls are likely to take Yorick 2 actions to dispatch, perhaps one if Vicious Blow or Brute Force are employed. If bad luck brings them out early, before Yorick has a chance to set up, they can be more troublesome, but no more so than a bad encounter deck draw. Their special ability is really only a problem if you are unlucky enough to draw them while engaged with one or more additional enemies
The discard condition: You kill them, which is something that Yorick is pretty good at doing. As usual, and investigator can deal with them, although Yorick rarely needs the help.
All in all, this is a below average signature weakness, generally offering only the mildest threat to the scenario.