“When the clouds above the covenant shape themselves into my sigil it is time to strike as the mundane guards will have been subdued.”
While drafting one spell to shape distant clouds a few more came to mind. The shaping spell might be used as a visual call to action which can be seen across a very wide area.
Conjure a Simple Cloud
Creo Auram 15, R: Sight, D: Sun, T: Individual
A cloud gathers high in the air, which will persist for the Spells duration reacting normally to the surrounding weather conditions.
(Base 2, +3 Sight, +2 Sun)
Theatrical Cloak of Black Tendrils
Creo Auram 25, R: Voice, D: Concentration, T: Group
Creates a voluminous and dark cloud of smoke which is filled with intertwining wisps and tendrils that surrounds the caster as they move. The cloud obscures all visibility within its core and then thins toward its edges; forming an opaque radius of 15 paces around the caster and thinning over 25 more paces.
(Base 3, +2 Voice, +1 Concentration, +2 Group, +1 unnatural locale)
An alternate for Rain of Stones (Ars p.127) which uses an acid instead of transformation to stone, with the General guideline for acidic air.
Rain of Viscous Acid
Muto Auram 20, R: Voice, D: Sun, T: Individual
Turns rain into a oily acid as it falls, causing +4 damage per round when it strikes an object or person. Continued exposure will cause significant damage.
(Base 4, +2 Voice, +2 Sun)
Rego Auram 20, R: Sight, D: Concentration, T: Part
A selected cloud mass is reshaped to suit the designs of the caster, such as animal shapes, letters, or three dimensional faces. The shape can be slowly changed but will disburse as normal when the spell expires. A simple Finesse check vs 3 is required.
(Base 2, +3 Sight, +1 Concentration, +1 Part, +1 complexity in shape)
This and over 584 other new spells for Ars Magica can be found on the spells page.