Board Thread:Suggestions forum/@comment-25012056-20181223225841/@comment-29620769-20190104185158

Orcs are not similar to elves, they have gone through some heavy modifying when they were turned from elves to orcs. This took unspecified amount of time, but we can assume it was long and painful process. (Also I don't think orcs go to the Halls of Mandos, and therefore also have very different kind of spirits)

However, they are probably already used to all kinds of dark magic, and would be much more resistant to that, which would result in morgul-blades not working on orcs with full efficiency