This section is kept purely for historical purposes.
The old version of this glyph allowed warlocks to boost their spell power by using Life Tap or Dark Pact. It combined with Fel Armor to increase spell power by 50% of Spirit, and Fel Armor helped heal back the life lost. With the Demonic Aegis talent, this could be increased to 59% (30% increase of the 30% provided by Fel Armor).
Useful for Warlocks of all specs, this glyph provided a near-constant boost in DPS from Spirit. Over the 40-second duration of the buff, Fel Armor healed 90% of the health used to trigger the Glyph, so effectively only 4% of health was used to keep this buff active.
If a Warlock had plenty of mana or could not afford to sacrifice a large amount of health (during PvP or while taking damage), a lower rank of Life Tap would still proc the effect (with significantly less health lost).
Patch 5.0.4 (28 August 2012): Changed from Reduces the global cooldown of your Life Tap by 0.5 sec.
/Patch 4.0.1 (12-Oct-2010): Changed from When you use Life Tap or Dark Pact, you gain 20% of your Spirit as spell power for 40 sec.
Patch 3.3.0 (08-Dec-2009): Now activates when Dark Pact is used.
Patch 3.2.0 (04-Aug-2009): Duration of the buff from this glyph increased to 40 seconds.