Shadowflame is a Warlock spell of the Destruction tree. It is a Cone AoE ability that affects multiple targets within its reach, dealing direct Shadow damage and a Fire damage-over-time effect to each.
Its primary use is in PvP, where, for most Warlocks, it serves as yet another instant cast damage spell, though it is a close-range AoE and care must be taken not to be within reach of dangerous melee classes such as Warriors. Its DoT is also effective for preventing Rogues from entering Stealth.
Boosted with the [Glyph of Shadowflame], the Shadowflame DoT also slows the target. However, the glyph has notable popularity only with PvP Destruction Warlocks, where it ranks 6th (wowpopular.com).
This spell truly shines in a deep Destruction PvP build, where Conflagrate will be available to provide extra burst as it can consume the Shadowflame DoT, which will in turn activate Backdraft for invaluable spell haste.
Patch 3.2.0 (2009-08-04): The angle of the cone effect on this ability was inadvertently very small, making it difficult to use. Its cone is now the same size as similar spells, such as Cone of Cold.
Edit
Patch 3.2.0 (2009-08-04): The angle of the cone effect on this ability was inadvertently very small, making it difficult to use. Its cone is now the same size as similar spells, such as Cone of Cold. 
