This skill is affected by Hell Gate.
Purgatory requires a lot of initial spirit energy, but the overall consumption of spirit energy, provided that it hit one target, is 1. Overall consumption is 0 with Soul Reaping's skill note.
Due to its high MP usage and spirit energy requirement, it is not recommended in PvP. However the damage is still remarkable (roughly half of the target's health).
Although Purgatory knocks down foes hit, you can use skills such as Energy Absorption or High Speed to continue combos, but doing so will reduce the overall damage of Purgatory. The reason for this is because using other skills while the orbs of Purgatory are still there will cause the remaining hits left by the orbs to completely miss. However, waiting until the orbs to completely disappear will cause the opponent to knock down.
By using High Speed before all the orbs explode, it is possible to do most hits and compensate for most of the neglected damage.