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Shadowform and spends its Discipline points getting to 
Mental Strength.
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1/2025300021203--550012401201301251The search runs separately for each fight and faction, and its answers disagree only on the cheap points listed under Flexible points.
Why these talents
Mind Flay is still a talent in Forever, and the build's most valuable one at 41.3 % of your damage: it is your filler channel, three ticks over three seconds. 
Improved Mind Flay, new in Forever, adds 20 % to those ticks and is worth 6.9 %.
Shadowform adds 10 % Shadow damage, makes Mind Blast, Mind Flay, Shadow Word: Pain and Devouring Plague crit for double damage, and halves the mana cost of every Shadow spell. The cost cut is why its worth grows with the fight: 16.4 % over one minute, 31.0 % over three, where the same rotation without the discount would run out of mana.
Darkness and 
Shadow Weaving each give 10 %. Darkness is a flat bonus to all your Shadow damage. Shadow Weaving adds 2 % a stack each time a Shadow cast lands, up to five stacks for 15 seconds, and with a Mind Flay landing every few seconds it stays on for 100.0 % of the fight. Darkness is worth 9.1 %, Shadow Weaving 11.3 %.
Shadow Focus gives Shadow spells 5 % hit and is worth 3.9 %. Pre-raid gear has so little hit that it takes your misses from about 15 % down to 10 %.
Improved Mind Blast takes half a second a point off Mind Blast's 8-second cooldown and is worth 3.1 %. 
Improved Shadow Word: Pain adds 3 seconds a point, one more tick each. One boss puts it at 0.8 %. Three bosses raise it to 4.7 %, because it saves a recast on each of them.
From Discipline, 
Twin Disciplines raises the damage of your instant spells by 5 %. In this rotation that means Shadow Word: Pain and Devouring Plague: 1.2 % on one boss, and more against three. Mental Strength adds 3 % Intellect a point, which buys crit, for 0.4 %.
Flexible points
- Blackout and Spirit Tap. Up to six points to get through the first row. Blackout's stun doesn't land on a raid boss and Spirit Tap needs a kill, so neither does anything here. If your Mind Blasts pull threat, move five of them to
Shadow Affinity and 
Shadow Reach: Shadow Affinity cuts your Shadow threat by 30 %, which the sim doesn't score. - Power in Light and Wand Specialization. On most fights, seven Discipline points for Smite, Penance and a wand attack the sim doesn't have, and the cheapest route to the 15 points Mental Strength needs.
Meditation, 
Mental Agility, 
Silent Resolve or 
Martyrdom serve as well, because mana isn't short: Meditation is worth 0.0 %. - Improved Mind Blast's fifth point. On three minutes the Undead build stops at four. The Night Elf's fifth point is worth 0.3 DPS, because Mind Blast ends up waiting for a Mind Flay tick either way.
Inner Focus. One free spell with 25 % more crit chance every three minutes, worth 0.2 % on a three-minute fight. Since 1 October its crit skips Shadow Word: Pain and Devouring Plague, so it goes on a Mind Blast.
Devouring Contagion cuts Devouring Plague's cost, which you don't need. Its other half, the plague jumping on when its target dies, never fires in a fixed-length fight.
Early Demise adds 30 % crit to Shadow Word: Death on a boss at or below 20 % health, the last fifth of every fight here. The one-minute build takes it for 0.3 %.
Vampiric Embrace is the point Shadowform requires. The sim never casts it. In a raid it heals your party for 20 % of your Shadow damage for 30 seconds, once a minute.
What each talent is worth
To price a talent, we take its points out and run the same fights again. Because talents feed one another, the column's total isn't your damage. The last-point column prices a single point, which is what matters when you move one.
On every fight
The one-minute single-boss build goes two points deeper into Shadow, 18/0/33. It adds Early Demise, Shadow Reach and a point of Devouring Contagion, and drops Spirit Tap and a point each of Improved Shadow Word: Pain and Improved Mind Blast. In Discipline it trades Power in Light and two points of Mental Strength for Silent Resolve and Mental Agility. On three bosses for three minutes both factions leave out Inner Focus: the Night Elf takes a point of Mental Agility, the Troll Silent Resolve and Martyrdom. Elsewhere the builds trade a point or two among Power in Light, Wand Specialization, Meditation and Improved Mind Blast.