Yes, these items are generally one-off constructions. They are very expensive (that is why the target in the backstory was a king) and need suprise to be effective. Once mages know what to look for, these suits lose their effectiveness. Go to Comment
In any case, I would let the GM decide on how well they are known. Their power is increased by their rarity. In this case, only Kagerith knows the way to make these suits (he has apprentices, but he is jealous of his secrets, so they only know part of the process). If his written works were stolen, then other mages could carry on with production. Go to Comment
Jeesh, the feedback others make to put a little more detail into the Bolt of Twilight has created a monster! :)
Seriously, when I started to read the description, I was wondering why people ever worship these things but that became pretty clear early on. This is an excellent alternative to the generic evil gods of death.