Yeah, as Super Dickmann said the whole scenario is weird in WC3 as is, even ignoring the entirety of the rest of the universe.
If ner'zhul threw the blade out, why is there a pretty pedastal with a warning? Did he have his minions go and make it seem like some heroic quest to trap arthas? Was he that convinced arthas wouldn't take the pedastal's warning seriously?
Was the guardian working for someone else? Revenants are already some weird undead-elemental hybrid thing that are immune to the lichking's control even in WC3 lore (wow seems to have made them mostly just fancy elementals and made them mostly irrelevant in wrath).