As a Shadow Priest, your role is to melt face! Nothing can stop your tremendous Damage over Time effects, so make sure to keep that Shadow Word Pain, Vampiric Touch and Devouring Plague rolling on your target. Mind Flay and Mind Blast are your go to spells, but keep in mind that good usage of other abilities such as SW:D, Shadow Fiend and Archangel is what will put your damage into the top ranks. Be vary of Mind Spike, as it clears all DoTs. In case your party is in real trouble, you can always help your party a bit with healing as well.
Recommended Keybindings for a Shadow Priest
SHIFT-3
[Archangel]
SHIFT-X
[Binding Heal]
SHIFT-2
[Cure Disease]
SHIFT-T
[Vampiric Embrace]
SHIFT-D
[Devouring Plague]
SHIFT-E
[Dispel Magic]
H
[Divine Hymn]
T
[Fade]
`
[Fear Ward]
V
[Flash Heal]
C
[Greater Heal]
X
[Heal]
2
[Holy Fire]
SHIFT-4
[Holy Nova]
CTRL-R
[Hymn of Hope]
CTRL-E
[Inner Fire]
6
[Psychic Horror]
CTRL-Q
[Inner Will]
Z
[Leap of Faith]
SHIFT-5
[Levitate]
SHIFT-A
[Mana Burn]
SHIFT-V
[Mass Dispel]
4
[Mind Blast]
F
[Mind Control]
CTRL-4
[Mind Sear]
CTRL-A
[Mind Soothe]
3
[Mind Spike]
CTRL-C
[Mind Vision]
SHIFT-Z
[Dispersion]
Q
[Mind Flay]
SHIFT-Q
[Vampiric Touch]
E
[Silence]
CTRL-G
[Power Word: Fortitude]
SHIFT-R
[Power Word: Shield]
CTRL-F
[Prayer of Healing]
R
[Prayer of Mending]
SHIFT-F
[Psychic Scream]
SHIFT-C
[Renew]
SHIFT-L
[Resurrection]
CTRL-S
[Shackle Undead]
SHIFT-G
[Shadow Protection]
5
[Shadow Word: Death]
SHIFT-S
[Shadow Word: Pain]
1
[Smite]
CTRL-W
[Shadowform]
CTRL-X
[Shadowfiend]