SPELLCASTERS : Spells over 6th level are extremely rare, and are never acquired by merely gaining a level.
Only the civilized kingdoms have clerics. They must be lawful and worship Mitra. There are priests of other gods, of course, and they may or may not have divine powers. Stygian and Hyperborean sorcerers can cast clerical spells and control undead if they are sorcerer-priests.