Zachary was in the City of the Dead in Nightmare, and when he got to that "cathedral" in the middle of the city, the arches weren't empty as usual, but still had their stained-glass windows intact. They seem to be regarded as "enemies", as Zachary's minions started attacking them. As you can see above, it's possible to Poison them, and the one on the far right is taking Fire damage from Zach's Act II merc, Haseen, who uses The Tannr Gorerod.

More screenshots of the windows: