ya there is a one block bump on the one side they are invisable on. i guess that ledge might be causeing a glitch? ill level that rest of the area out and see if they stop turning invisable. they do just walk right past my defences with out being shot at and dont appear till they hit me or the crystal. that was annoying as all hell.This is awkward because i've got through the whole event without an invisible monster a t all. Do you have a flat surface?