the main point is Plague bolt base is way worse than Void Bolt.
Once traited Plague bolt is nice...but Void Bolt gets 1.5 second casting. THe main point is now you get 25% damage when the target has 5 deathly callings on it. Lets compare the two:
I'll use both rank 1 and rank 10 to show just in case there is better scaling for one than the other. Plague Bolt 2 is considering if you have 25% bonus from deathly calling stacks.
Rank 1:
Void Bolt: 18-20, avg: 19
Plague Bolt: 13-15, avg: 14
Plague Bolt 2: 16.25-18.75 avg: 17.5 Remember rift rounds up and down so its really 16~19 avg: 17.5 aka 18.
Rank 10:
Void Bolt: 97-101 avg: 99
Plague Bolt: 73-76 avg: 74.5 aka 75.
Plague Bolt 2: 91.25-95 avg 93.125 aka 93
So in terms of damage with deathly calling plague bolt does ALMOST the same damage. But since void bolt has a quicker cast time it is still superior. Now I'd say it would be still be a decent idea to start the combat with a traited plague bolt shot to start combat out with 2 stacks. Then you can easily work with graverot to max it out for any pet abilities.
The main reason for wanting as many stacks of DC as possible is one thing: Revenant.
Revenant uses DC stacks very quickly and any enemy in range that does not have DC will not be damaged and debuffed with ruination. How does Plague bolt help this? Well it at least assures the main target, or a few if you rotate between them, have enough stacks. Revenant can do amazing aoe damage when 3+ mobs are affected by Necrosis. Other than that graverot should give enough DC stacks to make you not need to many for pets who consume stacks.
I use graverot because Knights aoe consumes 1 per enemy aggro'd, therefore I need aoe DC...therefore I need to use graverot

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