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Keywords are:  'necromancer spell soulforge forge'

Spell: soulforge                Mana:  15  Int: 20  Wis: 18  Lvl 15 Necr  (28%)
                                (helpful)   Requires: soulsteal
                                Group: Necromancer Soulbind

The soulforge spell is used to merge low level soulstones, destroying
one of them and raising the level of the other.  Soulforge is most
effective when merging stones of similar level, though repeated
merging of low level soulstones will gradually raise the level of
even the most powerful stones.

The order of the stones does not matter at all.