When you Shadowbolt, Incinerate or Soul Fire a target that is at or below 35% health, the cast time of Soul Fire spell is reduced by X% for 10 sec. Soul Fires cast under the effect of Decimation cost no shard.
When you Shadowbolt, Incinerate or Soul Fire a target that is at or below 35% health, the cast time of Soul Fire spell is reduced by X% for 10 sec. Soul Fires cast under the effect of Decimation cost no shard.
Decimation is a warlock talent that reduces the casting time of your Soul Fire when you hit a target that is at or below 35% health with Incinerate, Shadowbolt or Soul Fire.
This talent is akin to Execute for warlock, and is mirrored in the Affliction tree by Death's Embrace which increases shadow damage done to targets below 35% health.
Although the cast time is decided the start of the cast, the soul shard is charged only when completed, so if the buff runs out before you finish casting a soul shard will be consumed.
Patch 3.3.0 (2009-12-08): Redesigned. When [Shadowbolt], [Incinerate] or [Soul Fire] hit a target that is at or below 35% health, the cast time of [Soul Fire] is reduced by 20/40% for 8 seconds. Soul Fires cast under the effect of Decimation cost no shards. The Decimation effect is not consumed when you cast Soulfire
Edit
Patch 3.3.0 (2009-12-08): Redesigned. When [Shadowbolt], [Incinerate] or [Soul Fire] hit a target that is at or below 35% health, the cast time of [Soul Fire] is reduced by 20/40% for 8 seconds. Soul Fires cast under the effect of Decimation cost no shards. The Decimation effect is not consumed when you cast Soulfire 
