Shadowfiend

In the Shadow Priest talent tree, Shadowfiend sits in the Shadow specialization talents, row 6 of 11. It is a single-point node. Its row opens once 8 points are spent in Shadow Talents.

Connected above

The tree draws a line into Shadowfiend from Shadowy Insight.

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  Shadowy Insight

  Shadow Word: Pain periodic damage has a chance to cause your next Mind Blast to be instant and ignore its cooldown.

Connected below

Shadowfiend leads on to Subservient Shadows and Mastermind.

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  Subservient Shadows

  Summoned minions last 20% longer and deal an additional 15% damage.

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  Mastermind2 ranks

  Increases the critical strike chance of Mind Blast, Mind Flay, and Shadow Word: Death by 4% and increases their critical strike damage by 20%.

Same row

Also on this row: Dark Thoughts, Maddening Touch, Improved Voidform or Ancient Madness, Shadeburst, Dark Evangelism and Shattered Psyche.

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  Dark Thoughts

  Increases the chance for Shadowy Insight to occur by 25%. When consuming Shadowy Insight, Mind Blast generates 1 additional Insanity.

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  Maddening Touch

  Vampiric Touch deals 20% additional damage and has a chance to generate 1 Insanity each time it deals damage.

• 
  Improved Voidform

  Voidform increases your spell damage by an additonal 5% and grants 2 additional uses of Void Volley.

• 
  Ancient Madness

  When Voidform ends, the haste lingers and decays over 10 sec.

• 
  Shadeburst
• 
  Dark Evangelism

  Increases your periodic spell damage by 6%.

• 
  Shattered Psyche

  Lasts 10 sec.