TL;DR: Sargeras can destroy entire solar systems with his penis.

Considering the size of Kalimdor based on /u/SwarlDelae comment from this thread and size of Sargeras in comparison to Kalimdor from MMO Champion, we can easily calculate the speed of his penis if he'd ever tried to do helicopter dick.

Let's assume, that Sargeras' height is 3/4 of lenght of Kalimdor, this gives us ~2500km.

Now let's assume, that penis length is 1/10 of our height (255km) and you can do two spins while doing helicopter.

This gives us a speed of 13c (speeds of light).

So, using the simple formula for kinetic energy, we can calculate, that even if it weighs 1kg, single blow with this speed would release energy equal to approximately 68 Tsar Bombs.

Edit: I took a quick nap and after waking up and sobering up I realized, I've used the wrong formula - circumference of a circle is π2r, not πr2. This means his peen is traveling at 1/300 of the speed of light. Still pretty impressive, but not as much as the original result.

On the other hand, if we assume that peen weighs 0,4kg and Sargeras is 13900000 bigger than a man his penis would weigh 5560000kg, so actual kinetic energy would be equal to ~97000 tsar bombs.