Soul Fire

Soul Fire (commonly misspelled Soulfire by Blizzard) is a mana efficient, high damage Direct Damage spell that consumes a Soul Shard. It is best used as your initial attack for pulling due to its long cast time. In this sense it is similar to the Mage's Pyroblast ability.

Talent improvement
Decimation will lower the cast time of Soul Fire by up to 40% for 10 seconds at max rank when you cast a Shadowbolt, Incinerate or Soul Fire at a target that is at or below 35% health, and will consume no shard while under this effect.

Bane will lower the cast time of Soul Fire by up to 2 seconds at max rank.

Tips and Tactics
This skill has been shown to be one of the most mana-efficient spells, but this is offset by the Soul Shard cost. Untalented it has a poor damage per cast time, so is not a good spell to use when damaging an enemy normally, but is very good as an opener when soloing where the cast time is 'free'.

If you have talent points invested deeply into the Destruction tree, this spell can deal huge amounts of damage to generic enemies and instance bosses when it critically hits and if you have a steady supply of Soul Shards then casting this spell two or three times on a boss will help improve your DPS.

For targets under 35% health, Decimation now allows Soul Fire to be cast faster, and without a soul shard. This talent makes Soul Fire similar to a Warrior's Execute.

The long cast time and Soul Shard cost of this spell make it harder to use in PvP as the time spent casting it is time gained for an enemy player to tear you to pieces.