What the achievement asks
Contribute 100 Radiant Remnants to any keyflames in Hallowfall. Five points, account-wide.
There is no list to work through and no coordinate to find. It is a counter, and it fills while you do other things.
Where the Remnants come from
Radiant Remnants drop from Hallowfall's world quests, events and general activity. They are not scarce, and the achievement is not really asking you to farm them: it is asking you to spend them rather than hoarding them.
World quests do not unlock until the main campaign is finished. If you have no Remnants and no way to get any, that is the reason, and no amount of flying around Hallowfall will change it.
Spend them where they do the most
Every Remnant counts here wherever you put it, so put them where they buy something else.
The eight lesser keyflames in the north-east take three Remnants each and produce a quest, which is Beacon of Hope and its seventeen quests. Lighting all eight every week is twenty-four Remnants, so four weeks of that alone finishes this achievement.
The large keyflames take up to twenty Remnants and stay lit for twenty minutes, which is what makes the bonus events of Sharing the Light available at all.
Three of the cats in The Missing Lynx only exist while their keyflame is lit.
There is no version of this achievement worth doing on its own. Anyone working through the Hallowfall branch passes a hundred Remnants without aiming at it, and anyone who hoards Remnants ends up with three achievements unfinished and a full bag.
Where it fits
One of the pieces of You Xal Not Pass, which rewards the Shadow of Doubt mount and is part of Worldsoul-Searching, the War Within meta.