Tri Spec Priest

The Focus of the Tri Spec build is to obtain four key Talents while picking up a few of the most useful Holy Tree Talents. The four Key Talents are Inner Focus from the Discipline tree and Mind Flay, Vampiric Embrace and Silence from the Shadow Tree. Vampiric Embrace is optional in this build as it could be swapped with a 4th point of Spiritual Healing.

Discipline Talents (21 points)

Unbreakable Will - 5/5 points
Increases your chance to resist Stun, Fear, and Silence effects by 15%.
This is used just to get up the tree.

Improved Power Word: Shield - 3/3 points
Reduces the duration of your Power Word: Shield's Weakened Soul effect by 15 seconds.
This allows the Priest to recast PW:S every 15 seconds. This Talent helps greatly when soloing.

Improved Power Word: Fortitude - 2/2 points
Increases the effect of your Power Word: Fortitude spell by 30%.
More health. Health is good.

Mental Agility - 5/5 points
Reduces the mana cost of your instant cast spells by 10%.
Decreased costs for PW:S, PW:F, Renew, Mind Flay, Silence, etc. More mana efficiency allows more heals and less downtime.

Mental Strength - 5/5 points
Increases your maximum Mana by 10%.
More Mana. With 7000 mana from base+equipment this adds a bit more than 9 Int worth of Mana per Talent point.

Inner Focus - 1/1 point
When activated, reduces the Mana cost of your next spell by 100% and increases its critical effect chance by 25% if it is capable of a critical effect.
A free spell every 5 minutes. Can cast even when out of Mana. Good stuff.

Holy Talents (8 points)

Improved Renew - 5/5 points
Increases the amount healed by your Renew spell by 15%.
More efficient Renews. This mitigates incoming damage to allow you to get off larger heals.

Spiritual Healing - 3/5 points
Increases the amount healed by your healing spells by 6%.
More healing. Healing is what Priests do. Could add another point here is you give up Vampiric Embrace.

Shadow Talents (22 points)

Blackout - 5/5 points
Gives your Shadow damage spells a 10% chance to stun the target for 3 seconds.
10% stuns from shadow spells can be useful. Gets you up the tree.

Spirit Tap - 5/5 points
Gives you a 100% chance to gain a 100% bonus to your Spirit after killing a target. For the duration, your Mana may regenerate at a 50% rate while casting. Lasts 15 seconds.
Some improved Mana regen when you occasionally get a kill. Could be swapped with a few points of Shadow Reach or Improved Mind Blast.

Improved Shadow Word: Pain - 2/2 points
Increases the duration of your Shadow Word: Pain spell by 6 seconds.
More damage from a single cast of SW:P. Increases Mana efficiency of a staple damage spell.

Shadow Focus - 3/5 points
Reduces your target's chance to resist your Shadow spells by 6%.
This helps to land spells on equal to higher level PCs/NPCs. Some research indicates that two points can reduce resists on equal level targets to almost zero.

Mind Flay - 1/1 point
Assault the target's mind with Shadow energy, causing 75 damage over 3 seconds and slowing the target to 50% of their movement speed.
Mana efficient damge plus snare. What's not to like?

Improved Psychic Scream - 2/2 points
Reduces the cooldown of your Psychic Scream ability by 4 seconds.
Allows you to fear more often. This helps in PvP, surviving multi-pulls, and in certain tactical situations in a group.

Improved Fade - 2/2 points
Increases the duration of your Fade ability by 10 seconds.
Allows Fade to be active 20 seconds out of 30. This gives the Tank more time to peel aggro off of you.

Vampiric Embrace - 1/1 point
Afflicts your target with Shadow energy that causes all party members to be healed for 20% of any Shadow damage you deal for 1 minute.
This is really an optional component of this build. It converts 20% of shadow damage you cause into healing for the whole group.

Silence - 1/1 point
Silence the target, preventing them from casting spells for 5 seconds.
A key Talent in this build. It allows you to interrupt a caster. Most mobs will charge when unable to cast so it is very useful as a pulling mechanism.