I checked WowHead and make sure the displayid on both npcs are correct. I thought they might have their modelid wrong but that is not the case. Here is the quest from WowHead. http://www.wowhead.com/quest=11523/arm-the-wards#comments
Apparently I have to loot something from the dead npcs in order to finish the quest and since they're not visible, I can't loot it. Tried as I might, I checked on both unit_flags and unit_flags2's flag descriptions but could not find anything that makes reference to being invisible. Guess I'm at a lost.
Tried with the most recent version TrinityCore rev. 1841752acb00 2017-06-05 01:36:04 +0200 (3.3.5 branch) (Win64, Release, Static) (worldserver-daemon).
Thanks for any other suggestion.