Most information and tactics that apply to the Mindbender also apply to the Shadowfiend - see [Shadowfiend] for more.
Has a base attack speed of 1.5 seconds, by default dealing 8/10 attacks per summon. However, the Mindbender benefits from your haste, granting extra hits and thereby increasing damage and mana generation. The Mindbender's haste appears to match the current haste of the priest, checked at frequent intervals. This means that the Mindbender will benefit from haste effects gained by the priest part-way through its duration.
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Described as an "improved version of Shadowfiend that is substantially more effective at dealing damage and generating mana or Insanity", the Mindbender relies on its 12/15-second duration and short 1-minute cooldown to outperform the Shadowfiend.
Deals 88% as much damage as the Shadowfiend.
Generates mana or Insanity per hit (Shadowfiend generates no mana, and half the Insanity Mindbender generates).
Available every 1 minute instead of every 3 minutes.
Has a potential uptime of 300%/375% that of the Shadowfiend, and generates a moderate amount of mana or Insanity per summon.
The Mindbender therefore has the potential to outperform the Shadowfiend considerably. However, this relies on the priest to make the most of its short cooldown. Additionally, it requires the priest to spend a global once per minute summoning the Mindbender (rather than once every three minutes) which is potentially a downside. One other advantage is having the pet active more often, which can help to distract or confuse enemies, and reduces the impact of having your pet CCed, since its time is less precious.
Patches and hotfixes
Patch 8.0.1(2018-07-17): Mindbender has been moved to the level 45 talent row (was level 60) for Discipline. For Shadow, Mindbender now generates 6 Insanity per swing (was 8).
Patch 7.2.5(2017-06-13): Mindbender now generates 8 Insanity per swing (was 4). Shadow only.
Patch 7.1.0(2016-10-25): Now returns 0.5% of your mana per tick (was 0.75%). Discipline only.
Patch 7.0.3(2016-07-19): No longer available to Holy priests. Duration reduced to 12 seconds for Discipline, and 15 seconds for Shadow (down from 20 seconds). No longer generates mana for Shadow priests. Additionally, [Glyph of the Sha] will no longer affect Mindbender.
Patch 6.1.0(2015-02-24): Duration increased to 20 seconds (up from 15 seconds).
Patch 6.0.2(2014-10-14): Now restores 0.75% mana per hit (down from 1.75% mana).
Patch 5.4.0(2013-09-10): Now deals 10% more damage, and mana return has been increased by 20%.
Patch 5.1.0(2012-11-27): Tooltip corrected, from 1.3% mana to 1.46% mana per hit.
Hotfix (2012-09-18):Mindbender damage has been increased by 20%. It now does 80% as much damage per hit as [Shadowfiend] (up from 66.7%).