An Evil Force

"Just as the Lord of all Terror brings life to nightmares, so too can he imbue inanimate objects with his vile presence. Culled from the desecrated churches, Stained Glass Windows can sometimes be found on the Mesas in Hell. These panes of colored glass are not what they appear, however -- they are actually alive! Don't worry, adventurer, they lack both mobility and knowledge of the arcane arts and so, they cannot attack you. Perhaps, they too are used to alert the demons of the underworld of unwanted visitors..."

- Deckard Cain

An Evil Force is a living monster found in the City of the Damned. According to Deckard Cain, these inanimate objects were imbued by the Lord of Terror with his vile presence. These stained glass windows can sometimes be found on the mesas in Hell. They lack both mobility and knowledge of arcane arts and cannot attack anything, however Cain speculated they are used to alert the demons of Hell of unwanted visitors. They are found only if a special "Cathedral" part of the City of the Damned spawns.

Trivia

 * It is among the rarest monsters in Diablo II, next to the Restless Spirit.