This quest was a bit confusing to me, so I'll explain it. Whenever you acquire [item=246771], you'll get a quest called A Radiant Echo. For me it didn't show on the map where I had to go and it wasn't even explained in the quest itself what to do.
Open up your map and look for a world quest with Worldsoul Memory in its name. Something like this [quest=82260], it will have some kind of circular azerite icon on it. There will be a few WQs like this, doesn't matter which one you choose. Go to one of these WQs and find [npc=222165]. Once you click on it, you'll be ported into a timed run. You'll only have a few minutes so make sure you're well prepared. Good luck!