Is it the key you find underwater with Bugenhagen and then take back to the City of the Ancients?
The Heliotrope bounty -
Heliotrope - The key looks like somekind of plant
Bounty - Its kind of like a treasure that a few people are after
Following tragedy most final -
Not sure what point in the game you find the key, but i think its after something big happens.
Descent gains potency -
Its underwater
Ascend on with the great chronicle -
Its the Key to unlocking knowledge about the ancients