I had the Exiled Mages prophecy too. There’s a large number in a circle to the top left (I think) that indicates how many enemies you have to kill. In my case, I had to kill 3 exiled mages to fulfil the prophecy once and get the reward. At that point, the prophecy showed 1/3 fulfilled.
I could then repeat this 2 more times, killing a total of 9 exiled mages to get 3 rewards.
Hope that helps.