Souls

Souls come in 4 different colors.
 * Green souls can be equiped to any piece of equipment and improve the stats of your character.


 * Red souls can only be equiped on weapons and provide such things such as bonus damage, increased critical chances, stun effects, and health (or mana) absorbtion.


 * Purple souls can only be equipped to armor and generally increase defense, evasion, or counter attack chances.


 * Blue souls can only be equipped to accessories and can cause increased drop rates, collection rates, stats, skills, and regeneration of stamina or mana.

Hell Mode Souls
These souls are dropped by any monster in the respective area, not all of the monsters seem to drop souls however. The monsters you would typically grind on do however.
 * It would seem as though the souls drop less frequently in the Cave of Darkness as they do in Stigia.