How often does Nightfall proc?

How often does Nightfall proc?

Nightfall looks like an awesome talent on paper but is actually not as useful as most people think. The bonus in dps you get from Nightfall is misleading. The global cooldown gives instant cast spells such as Nightfall Shadow Bolts an effective "casting time" of 1.5 seconds. Since regular Shadow Bolts take longer than 1.5 seconds to cast, they are not affected by the global cooldown. If you are Shadow Bolt spamming, you are effectively only cutting that cast by 1 second (2.5 - 1.5 = 1 sec).

There are two formulas to calculate how often you can expect a Nightfall proc. 1-(1-p)^n is the reduced formula to find the probability of at least one event in n trials. The chance to get at least one Nightfall proc in one cast of Corruption is therefore:

1 - [(1-.04)^6]
1 - (.96^6)
1 - .7828
.2172 = 22%

The expected number of Nightfall procs in one cast of Corruption is:

6 * .04 = .24

It's better not to consider Nightfall as any reliable part of your dps. Rather, it works perfectly in the context of the Affliction tree as a small bonus to your dps since you'll have Corruption up at all times and on multiple targets, and will often be draining life.