Apparently you kill the 'corrupted creatures' and they dropw this Soul Shard.

I've collected most of them from Felwolves, the drop is maybe 1/3 or a tad higher.

I killed two bears, none on them.. this doesnt mean they dont drop on the bears, just that I didnt loot any off the bears.

Gives roughly 4400XP and the Cenarion Plant Salvex2.  It's a reoccuring quest, not sure if it gives 4400 each time. Most likely will give less.