Many spells, especially divinition spells require the invocation of certain spirits. Well, this can be good and bad. For the beginner, i'd advice against it. Amongst the dangers of spiritual invocations are possesion, where the spirit takes hold of your body. Although most spirits are virtually harmless, there are those who, over the years have gotten their own brand of magic, and it is there spirits you have to watch out for.
The ancient art of summoning spirits is known as necromancy. In some places, experienced necromancers summon spirits to posses them temporarily to get in contact with the "other side".
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