A spell I wrote up a long while back – Melt the Vanguard’s Arms is useful against one target. This is a quick modification to that spell to affect an entire Roman company of up to 100 soldiers.
Melt the Company’s Arms
Creo Ignem 35 / Terram, R: Voice, D: Mom, T: Group
Up to ten metal items carried by or on the target are heated to melting point. Thus a soldiers sword, helm, and segments of armor will all instantly be burning hot, and melt. This has a side effect of inflicting +5 damage per round while metal items targeted are carried or worn.
(Base 10 heat an object to melting point, +2 Voice, +2 Group, +1 increase size of group, +0 Te Requisite)
The spell guidelines in the first version used Target Part and an additional magnitude to affect multiple parts, however when affecting 100 targets the Target: Group makes more sense and would incorporate the needs of Target: Part.
More custom spells can be found in the New Spells for Ars Magica page.