The Ghosts in Eris' fireteam either died from the lack of connection to Light, or had the Light stripped from them by the Hive.
Darkness Zones are actually mentioned in the Grim under the Cabal cards and seem to be areas where the Darkness is heavily concentrated, so probably high death areas. In these areas, Ghosts require Light from another Guardian to help revive. There's no telling what happened to the Ghosts in Twilight Gap or Battle of the Six Fronts as those most likely became Darkness Zones.
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Not sure i could be convinced temporary dark zones were created during six fronts and twilight gap. Otherwise dark zones could explain eris's raid team. My theory is still plausible. So far its the best ive heard to explain the battles. I dont believe the fallen can do more to us to kill our ghosts than we do to ourselves in the crucible.