Demonic Sacrifice

Demonic Sacrifice was a warlock spell available via talent that sacrificed a summoned demon to grant a powerful buff that lasted 30 min. Each warlock demon granted a different buff when sacrificed. The effect was canceled if any Demon was summoned.

Imp: Increased your Fire damage by 15%.

Voidwalker: Restored 2% of total Health every 4 sec. Changed from 3% in 2.0.

Succubus: Increased your Shadow damage by 15%.

Felhunter: Restored 3% of total Mana every 4 sec.

Felguard: Increased your Shadow damage by 10% and restored 2% of total mana every 4 seconds.

Strategy
The idea is not to use this spell as an emergency spell (compared to the standard Voidwalker spell Sacrifice), but rather for a completely different strategy where you don't use your pet.
 * Sacrificing the Imp (15% Fire damage) is only effective for Warlocks with heavy talents into the Destruction tree. The spell damage of Incinerate is greatly increased by sacrificing an imp. Demonic Sacrifice on an imp plus Destruction talents is a very effective "nuking" build, allowing warlocks to rival even a mage's burst damage.
 * Sacrificing the Voidwalker (2% of total health every 4th second) is probably the most interesting variation. One reason is that most Warlocks have equipment that gives lot of stamina. This is an interesting strategy in boss fights and Battlegrounds. It is also effective with small groups where you have a tank, which means you will have no down time.
 * Sacrificing the Succubus is comparable to actually having it kept alive (+15% shadow damage). Can give a really nice boost of damage if you are for example high affliction and dot/life drain.
 * Previously, sacrificing the Felhunter was generally not as good as sacrificing the Voidwalker, as only 2% of mana every 4 seconds was regenerated. This has been increased to 3% per every 4 seconds. Although it may be better in certain situations to regenerate health and Life Tap that health into mana, Life Tap triggers the global cooldown and can reduce your overall damage output. Sacrificing the Voidwalker is effective in encounters where you expect to take periodic damage over an extended duration, such as certain boss fights.

Note
It is not possible to have the Demonic Sacrifice active at the same time as Master Demonologist (changed in patch 1.12).

Regeneration of hp will add threat for you. That means that sacrificing the Voidwalker in situations where aggro is sensitive can be dangerous.

It is not unusual to find discussion on the meaning of a talent tree where you improve the pets a lot, while at the same time set talents only active when the pet is sacrificed. One motivation for this is that there are situations where it is not possible to use the pet. In those cases, Demonic Sacrifice provide a back up for the Warlock who would otherwise lose much of his powers.

It is highly useful to use this Talent when Enslaving a demon, as enslaving demons does not count as summoning a new minion and will allow you to have a pet and this buff.