In this scene, the group has discovered some cultists, doing some sort of ritual magic behind City Hall. (If the cultists are allowed to complete the ritual, it improves their combat abilities: they get additional fire damage on attacks, resistance to fire damage, and anyone who strikes a cultist in melee takes fire damage from their burning blood.)
If left undisturbed, in ten minutes the cultists will finish their magic and then march to the museum, with the goal of claiming all the artifact weapons they can find. If allowed to do this, the weapons will cause them to turn on each other, and the survivors will rampage through the village causing senseless destruction and murder.
The group could handle this by simply removing the artifacts from the museum. But these cultists are bad people, or at least extremely dangerous fools, and it's probably best if they're killed or captured.
This is a difficult fight, because all of the cultists have fire magic which they project from their draconic symbols of Tiamat. (If you're playing 5e, you could model this by letting them cast burning hands at will. Note that the cultists have no special protection from fire.) Brother Sunwyrm, the highest-ranking cultist, has additional fire magic at his disposal -- narrate a fire shield, or a ball of fire, or whatever else seems to fit the situation.
The group has three ways to even the playing field.
The group is aware of the cultists, but the cultists are focused on their ritual and not aware of the group. Perhaps the group can concoct some sort of surprise attack.
The group can defeat a cultist in one roll by simply taking their holy symbol, which removes all their access to magic. (The cultists have daggers as well, but they're not any good with them.)
The third, and most interesting, approach is to use the artifact weapons against the cultists. The artifacts are tremendously powerful, but might turn against the group at any time. Note also that the cultists can attempt to take the artifact weapons using the normal disarming rules. This approach is safest if the group has access to some way to maintain control over the artifacts.
If the group survives this battle, they're probably going to want to think more about what to do with all the artifacts they've captured.