Passive Souls, as their name suggests, grant base stats or an ability that applies as part of a large action or automatically throughout the game. Each player can carry a total of 4 passive souls but there are two passive soul slots that take priority in placing the soul in your inventory. On the other hand, each player can only carry two active souls which have their own assigned item slots. (Num. 4, Num. 5)
List of passive souls (notation of x / xx / xxx refers to having rank1 / rank2 / rank3 of the soul):
If damage taken in a single attack is 250 or higher, automatically activate mystic guard preventing the damage and all further damage for 1 second. This will not trigger mystic guard's cooldown. Has a 65/55/45s cooldown.
Enemies that deal damage to you from any distance will take 70 damage. This effect has a 6/5/4 second cooldown. On death you will explode, dealing 500/750/1000 damage in a large area. The death effect can only occur twice per boss fight.
Greatly lowers the likelihood that you will be targeted by boss abilities and attacks and increases attack speed by 12/18/24%. (At rank 3 this sets your AA swing timer to T/1.24, which is 1.21s for ranged and 1.61s for melee)