Are Magica magical devices are activated by a triggering action, and also can have linked triggers. Simplistically this means that many effects can be daisy-chained together to form a more complex combined effect with several enchantments. What I’ve rarely seen is a way to make an effect automatically be cast each round for many rounds. It could just be that it’s not be written up, or that I’ve missed it completely. Here is my take:
To do this a “ticking” spell needs to be invented, which takes a prescribed action each round while it is activate. This could be as simple as a Rego effect which moves a switch between two points each round for it’s duration, or the effect depresses a button already set into an existing item. There needs to be a way for the ticking effect to know which thing to move, which is either specified by the caster when the spell is invoked, or set when the effect is invested as part of a magical item.
Then a second effect (say a harmful one, like a nasty area of effect fire spell which does not target the small-ish area where it is invoked) is cast each round by the magical device, with the caster only needing to concentrate on the ticking spell. This is made even more effective when the caster of the ticking spell (which I’m calling The Unseen Fidgeting Hand) passes the burden of concentration to a spell like Maintain the Demanding Spell which will maintain concentration for them for two minutes.
I have two example combat spells to be imbued in an item in mind to make the nastiness of this approach clear; briefly either:
(a) Creo Ignem effect bursting fire around the caster as Ignem is excellent for broad brush devastation, particularly when more effort is put into the device to make it penetrate magic resistance.
(b) A version of Vilano’s Sling which throws a large amount of stones around the caster, so that any target does not benefit from any magic resistance they might have (written up as a spell here). The sling spell could also easily be a Rego Herbam / Terram based effect to throw arrows just as easily.
The Ticking spell written up is as follows:
The Unseen Fidgeting Hand
Rego Terram 5, R: Voice, D: Concentration, T: Individual
This is a general spell for automatically moving a device or thing in a normal manner as directed by the caster, including moving or using the device repeatedly. The intent is that the device will be used once per round without the caster needing to directly concentrate on the action, for as long as the spell is active; for use with other enchanted devices which have manual triggers.
e.g. opening and closing a latch, turning a key repeatedly, throwing a lever, pressing a button, switching a toggle, or a similar effects. A soft tick is heard every round the spell is active as a free cosmetic effect.
It is invented as a Rego Terram effect to utilize the general spell guidelines for moving general items. Casting requisites may be required.
(Base 1, +1 for stone, R: +2 Voice, D: +1 Conc, T: Ind)
Effect modifications for enchantment (+5 maintains concentration, +10 unlimited uses)
The Unseen Arm (Ars core rules, ReTe5, p 155) can do this too, however the caster would need to concentrate each round on The Unseen Arm for each action which saves no real effort.
These and many more custom spells can be found in the New Spells for Ars Magica page.