The cell key 2 is a quest item only used in the Troll Stronghold quest. It opens the cell where Eadgar is held. The key is held by Berry, a level 71 troll sleeping in guard in Troll Stronghold. It can be pickpocketed from him if player has level 30 Thieving, if not, players must fight him to get it.
For an exhaustive list of all known sources for this item, see here
|10 December 2015
(update | poll)
The examine of this key has been updated from "The key to Mad Eadgar's cell in the troll prison." to "The key to Mad Eadgar's cell in the troll prison (Used in the Troll Stronghold quest)." so its purpose is better described.