Covenant with Eden 'insteads' any removal of characters so that they are discarded.
So, when a character would be removed from the game, they are 'instead' discarded. If you have another card like Chamber or Potter's Field or the like that 'instead' a discard, the new effect (discard) 'instead' causes the character to be placed in the fortress. These cards do not 'remove from game' (which is its own effect that takes the card out of the game permanently, not just out of play), so they are not affected on the front end.
Basically, if you had Covenant and Chamber out at the same time, if an effect caused your angel to be 'removed from the game' then 'instead' it would be discarded by Covenant, and then it would 'instead' be placed in Chamber. It ends up in Chamber when everything finishes resolving.
EDIT: Instaposted
