Сan be used for Elixir of Heroism.
This item can be found during the quest No Rest for the Wicked. After killing Velasco and his gang in the Castillon's Landing area, Isabela suggests you look around first before continuing on; look for the item in a chest in the northwest room.
The flower is rare, and is in danger of becoming extinct because of its value in the creation of ambrosia, which is distilled from the roots of the plant. Dozens of these plants go into the making of just one vial. Some say that the wives of the most powerful Tevinter magisters once used ambrosia to perfume their baths in a vulgar display of wealth.—An excerpt from The Botanical Compendium, by Ines Arancia, Botanist
—From Codex entry: Ambrosia