Repairing the white phial quest broken?

Solution 1:

The briar hearts in this quest are apparently prone to glitches. I found this thread where people are discussing the various ways in which they were able to break this quest. In some cases, when they got notification that they needed to get that specific briar heart, all the briar hearts they already had became quest items.

I think the best you can do is load an earlier save, or hang on to this quest until Bethesda patches it to work properly. Even so, you might not be able to finish it unless they create some way to reset your progress in it or otherwise fix the quest if it is already in progress.

Solution 2:

If you can locate the specific corpse they want you to find, try placing a briar heart in the body, closing it, reopening it, and taking the heart. You can source the spare briar heart from another Foresworn, an item placed in the world, or the randomly-generated ingredients at an alchemist.

Solution 3:

A possible workaround from UESP Wiki, "Repairing the Phial - Bugs" article:

If you have initiated the quest to find Red Eagle's sword, the same Forsworn Briarheart might be the target of both quests. It will carry both Red Eagle's sword and a briar heart. However, even if you pick up both, it will only complete the Red Eagle quest. You will need to find a different Forsworn camp to complete the briar heart part of this quest.