From Darkness, Comes Light

From Darkness, Comes Light is a priest talent, available at level 45. It consists of two distinct effects: Surge of Light and Surge of Darkness. The former is helpful for priests who are healing and/or using Smite, while the latter is useful only to Shadow priests.

Surge of Light
This effect gives a 15% chance for your Smite, Heal, Flash Heal, Binding Heal, Greater Heal, Penance, Circle of Healing, Prayer of Mending and Prayer of Healing to grant a free, instant Flash Heal. This has the obvious benefit of saving mana, but can also be useful in emergencies, PvP and situations where healing is required while moving. The effect has a 20-second duration and can accumulate up to 2 charges, allowing the priest to choose the right moment to use these procs.

Although primarily useful for healing priests, Shadow priests can still benefit from this effect when casting any of the related spells.

Surge of Darkness
This effect grants a 20% chance for your Vampiric Touch ticks to empower your Mind Spike. When empowered in this way, Mind Spike is instant cast, costs no mana, has 50% increased damage and no longer removes your damage over time effects from the target. This makes it possible to include Mind Spike in a DoT rotation, increasing DPS and providing a powerful instant attack that can be used on the run. The effect can accumulate up to 2 charges, but has a limited duration of 10 seconds in which to make use of them.

Mind Spike procs from Surge of Darkness increase Shadow priests' mobility, burst capabilities and overall DPS, especially when compared to the Mind Flay usually used to fill the rotation.