I think that spells like Havoc’s Hand (pg 214 BWG) can be sustained and used from one target to the next, each target only requiring a successful touch to activate the spells damage. It is possible to create a spell that would do continuous damage such through Abstractions and Distillations (Maic Burner). Once cast, such a spell would continue to damage anyone and anything that remained within its area of effect. (Try adding sustained to Fire Fan and see what happens)
Havoc’s Hand creates a weapon that you need to sustain in order to keep around. Every hit is a new roll for injury.
Creating a spell that did continuous damage would be possible though as per Let It Ride would not get to re-test. Similar to the Bridge of Death in Thelon’s Rift, anyone who successfully beats the bridge ob is able to take a nap on it without any problems. With a spell of poisonous gas that was Power = 1/2 Will + Successes per volley, you’d roll once at the outset to set your power and that would be that.
I’m pretty sure there’s a bit in either the Adventure Burner or BWG about getting set on fire but I can’t find it right now. I think it follows what I said up there (test once, apply periodically until dealt with) but I’m only about 95% sure
I was considering creating something akin to the Elder Scrolls Flame Spell. It would create a gout of fire like in Fire Breath that would have the sustained facet. Once cast, the spell would cause searing jets of fire to erupt from the casters hands, for 10s paces length for sustained duration doing 1/2 will plus 2 damage (VA 2) Longest Weapon length, 1D Range Dice.
Meeting the obstical starts the fire burning and sets it’s parameters, but you would still have to direct it. Remember, the flames are comming from you and move as you do. (Perhaps a Directed Spell skill should be opened for such circumstances?)