Shadow Priest
Rotation
Opener: Vampiric Embrace, Shadow Word: Pain, Mind Blast, Mind Flay.
Priority: Keep Shadow Word: Pain up, Mind Blast on cooldown, Mind Flay as the filler; Shadow Weaving stacks to 5 for the warlocks. Devouring Plague once a fight if Undead.
Cooldowns: Silence for interrupts, Psychic Scream to peel, Shadowform always on.
Forever notes: The Nov 2 damage pass helps Shadow, and Forever's raids are 10- and 20-player, so a Shadow Priest finds a seat more easily than in Classic 40s.
Stats
Spell Hit to 16% (10% with Shadow Focus), then Shadow spell power, then Spell Crit, then Intellect, then Spirit (Spirit Tap).
Shadow Weaving means the whole raid's shadow damage benefits from your uptime; stay alive and keep casting.
Numbers for every spec and a gear comparer: stat weights.
Where it ranks
See the tier lists for PvE and PvP placement, and Classic vs Forever changes for what Blizzard altered in this class.
Wowhead and Icy Veins keep spec guides of their own: Icy Veins Shadow Priest, Wowhead class guides. Our rotation notes are editorial, written for the Nov 2 tuning.