'mounted spellcasting'

This automatic skill is checked every time a magic-user attempts to cast a spell while mounted on the back of another creature (horse, dracolich, dragon, whatever it may be). If unsuccessful the caster will be unable to cast the spell correctly and loses concentration. A magic-user can cast spells while riding even without this skill, but the chances of losing concentration will always be high. It is not until this skill is practised to sufficient proficiency that mounted spellcasting will become a sure thing. Clerics and knights do not require such a skill to have their god given powers castable while mounted. Mages and outcast knights all gain this skill for aiding in their casting while riding. Rangers, who have few spells, do no learn this skill and are best off casting dismounted.