There are two main types of conjuring spells. Those which conjure objects and those which conjure spirits.
Spells which conjure spirits are relatively simple to cast and rarely have negative consequences to casting.
However, those spells which conjure spirits are some of the most powerful spells in existence. Only a true caster can cast these spells correctly. When these spells are cast incorrectly the caster is usually haunted for the rest of their lives by the tormented soul they tried to conjure.