These groups spells aka. rituals could be kind of spells but these would
require several casters.
Rituals could be casted for example by five tarmalens. Performing a ritual
would require that casters would cast ritual in same room almost same time
(perfect synchronization is propably impossible(?)).
Other possible requirements for succesfull casting:
Casters: Correct number of casters (who can cast ritual), note also
combinatios eg. 5 tarmas + 1 evil priest + mage of electricty etc.
Time: for example midnight
Ingredients: what ever
Place: for example in some stonehenge like ritual place
Requirements of course would not be same for all rituals. For example some
rituals could be casted without ingredients and in any place. And the greater
efects more greater requirements. Some rituals could also have negative efects
if they failed.
In order to cast rituals caster should learn new spell: ritual (could also be
skill). So there would be only only one new spell but it could be
parametrized. If there for example would be ritual which could summon sharon
to batmud it would be casted following way:
Casters are gathered in correct place in correct time etc. and type \'cast
ritual summon sharon\'
And if everything went correctly, sharon would be summoned in world.
Evil priests\' rituals effects would of course not be so nice to other players
Examples of possible rituals:
Summon charon
Triggering some event
Casting exp rate boost over party
Ritual which could cause n% of monsters turn in to aggro
Insert your idea here
Note that ritual names and requirements would not be public information but
they should only be given when caster advances correct level in guild,
finishes some quest etc.
How does it sound? Any critic and ideas are welcome but please no flames

I
think that these rituals could give some extra spice to game.
And if you got this far thanks for reading
Clerion