Your combat loop
Press the first one that is ready.
Use Wrack. It does 18% more than Shadow Bolt.
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Life Tap When mana runs low
Use it when your mana runs low.
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Corruption Keep on target · 18 sec
Keep it on the target; it lasts 18 sec.
18% of your damage -
Bane of Agony Keep on target · 24 sec
Keep it on the target; it lasts 24 sec.
16% of your damage -
Siphon Life Keep on target · 30 sec
Keep it on the target; it lasts 30 sec.
6% of your damage -
Wrack Filler
Cast it whenever nothing above is ready.
61% of your damage
The first 30 seconds
When to press each one from the pull. The blue line under a press is the global cooldown (1.5 sec) or cast it holds: nothing else on the global cooldown can go until it ends, so of two that are ready together the higher one goes first.