mysterious darkshrine problem

So, i found a darkshrine with the effect "An unlikely guard for such as valuable treasure"... It is supposed to make one of the enemies in the current area drop an unique item. The problem is, i killed everything in the area prior to it.
I wasnt sure if waiting for a new instance(in the labyrinth) was a waste of time so i left but it really is a bummer.
Should i have waited for a new instance?

If you killed everything nothing can be done, its not like you'll get shav's or idk what out of it anyway