This is a quick spell to burn a building down from the inside. In the design I wanted an effect which would allow a pyromancer to immolate parts of a building but not others. Picture a staircase and rooms burning, which directs the panicking inhabitants down the only “safe” passage. Nasty!
It means adding a dual-target adjustment in the spell design, to grant the flexibility and control desired. I thought about creating the spell as Diameter instead, but thematically liked the idea that the magus can at will cancel the effect, and once several rounds have passed the building will continue to burn even though the spell has expired.
Immolation of Harrenhal
Creo Ignem 40, R: Touch, D: Concentration, T: Structure
Heats an entire structure, or a sub-part chosen by the caster, and everything within it to melting point, which creates a devastatingly dangerous environment for the inhabitants and materials. All items including typically non-flammable items will catch fire, and likely continue to burn after the spell expires.
(Base 10 to melt an object, +1 Touch, +1 Concentration, +3 Structure, +1 Part flexibility)
This and over 505 other new spells for Ars Magica can be found here.