Originally posted by Zep Tepi:

Oh yeah, you walked into a spawner nest. No wonder you had problems! Next time listen for the sound of flies. That means a spawner is near. You can take them out with arrows, but they will spawn nasty stuff when they are damaged so either proceed with caution, or leave them a lot of room. Usually spawners are in small stone circles or in stone ruins.

…

Just to add:
There are also Draugr Spawner in some abandoned Villages in the Meadows.