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 20/40% for 10 sec. Soul Fires cast under the effect of Decimation cost no shard.
Patch 5.0.4 (2012-08-28): Redesigned. When you cast Soul Fire or Shadow Bolt on a target below 25% health, you gain the [Molten Core] effect. Icon changed to
Old: When you Shadowbolt, Incinerate or Soul Fire a target that is at or below 25% health, the cast time of your Soul Fire spell is reduced by 20/40% for 10 sec.
Patch 4.0.1 (2010-10-12): Health requirement reduced from 35% to 25%. Soul Shard note removed.
Patch 3.3.0 (2009-12-08): Redesigned. When [Shadow Bolt], [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 Soul Fire.
Old: When you Shadowbolt or Incinerate a target that is at or below 35% health the casting time of your next Soulfire is reduced by 35/60%