Death is the most extreme condition within Godwars and can happen either from bad luck or letting a -3 or lower health condition drop to death over time. You may also SLAY a follower if you find them useless or not fitting in with your follower mantra.
Resurrection stones may bring back a DEAD follower. The exact odds of a stone are 1:10,000 Loot Drops. They can fetch a high value in the market or you can hang on to it for an emergency later on.