Since power flails is probably not a realistic thing for humans to have access to, i present you with an alternative to flails, a flaming censer that adds fire dots to the targets struck by it.
Something like this:
But with a heavier chain and more jagged edges, you can say its filled with purifying promethium or oils that splashes onto targets that are struck by it to justify the fire dot.
It fits in with the whole religious “purge it with holy flame” theme the preachers has going for them, its also somewhat lore accurate i believe.