A little-known fact about wither skeletons: If a wither skeleton is armed with a bow, all the arrows it fires with it will always be set on fire. This seems to be an innate property of wither skeletons, as this occurs even if the bow is not enchanted with Flame.

I suspect that at one point during Minecraft's development, these "wither skeleton archers" were meant to appear in some context, perhaps in nether fortresses alongside the sword-armed infantry. Perhaps these archers were removed later, deemed too powerful for normal players, and replaced with the regular skeleton archers that now populate the fortresses.

Anyway, this datapack reverses that. Any time a skeleton is spawned on top of nether brick, it is replaced with a wither skeleton armed with a bow. For obvious reasons, this makes the fortresses slightly more dangerous, but it also makes the already somewhat rare wither skeletons slightly more common, and more interesting, so I guess it all balances out.