Reminds me of tire fires that start and go for years, one started in 1989 – In Heyope (near Knighton, Powys, Wales) and 15 years later in 2004 it was put out. so even in our possibly low magic world there are fires that burn for over a decade...
That was kind of my thoughts. Some unusual magic of the world keeps it burning, and the Raven Sisters, being one of the few groups who cares about the wellbeing of the land, is trying to figure out why.
I haven't filled it in more than that because i tend to not write up more details unless my party chases the legend, because they might just move on and leave it (they did that with The Hollows). They got involved with the conflict between leaders, but left without really researching the graveyard in the slightest.
Sometimes players get curious about the strangest things.