Magi of old
Ancient magi who wield magic from the time when they were alive, They exchange their low physical stats for the ability to wield magic powerful enough to trash any modern magus, coupled with powerful and unorthodox Noble Phantasms make them more then able to be a player in the war.
Example Heroes: Gilles De Rais, Medea
To force the servant you are summoning into the class of Caster you just add these three lines to the end of your summoning chant.
"Thine words have carved a path of destruction.
Thy magics have challenged the boundaries of progress. I am he who shalt conquer thy knowledge. "
As listed on the Servant Category Page
Their free Class skills are:
One of the following: Thaumaturgy / High Speed Divine Words / Rune Magic / Sacraments Rank D
Casters are then allowed to pick up to seven skills for up to 37 points.