Perpetually Burning Terrain

I just played a Sidar game and I was next to Einion Logos of Elohim and at some point he converted to Ashen Veil (not from me). Also, Hyborem entered near both of us.

Einion's lands started burning and turning to hell terrain. That much I expected, but the strange thing is that the fires didn't go away in a certain spot. The deserts in his lands just kept burning. There was a good 3x10 chunk of land that just kept on burning for the rest of the game. It was desert, so it wasn't burning trees.

Is this a bug? I've never seen such a large chunk of land burn like that.

Also, I noticed that the hell terrain kept spreading even after I destroyed Hyborem (the hero AND the civ). Is it just because of the Ashen Veil religion, or what?

hell spreads hell, so you wold need to sanctify all the ell in the world to stop it completely. the hell version of desert is called burning sands.

Burning Sands generates a lasting blaze after a while. It can hinder an invasion quite a lot if Infernal started in a desert. In addition to Sanctify, Spring (or promoting magic/fire resistance) from adjacent square can allow you to douse the flames long enough to move troops through or siege a city.

Orcish and Demon units start with enough fire resistance to move through fire of course.

Solution?: Sanctify the hell out of it.

Couple more things about sanctifying:
- The Infernal lands always reset back to Hell Terrain automatically at the start of every turn IIRC. Burning Sands still takes a little while to start burning afterwards, but otherwise the normal spread timer doesn't apply within their cultural borders.
- The hell terrain spreads to ice, ocean and mountain tiles even if they don't change appearance. You might be unable to reach those if you're trying to clean the world of it. From there it'll gradually spread back to Sanctified lands.