Shadow of the Warp: How effective?

I've had the math done on Shadow of the Warp for awhile. This rule is crazy nice against psykers and you really get your bang for the buck with it being always active within 12'' of certain monsters. Since Flyrants and Tervigons are staples of current Tyranid armies you'll likely have several units with this rule.


My calculations here are all done using a MEQ Librarian, Ld 10 wizard. Obviously if the Psyker's leadership is lower Shadow of the Warp only grows in power but for now we'll assume MEQ Wizard.

The first thing that Shadow of the Warp does for you is cause a high failure rate for psychic powers. On 2D6 you have a pass rate of 11/12 or 92%. That's pretty awesome, for anything in this game. With SotW you're now making that test on 3D6. Your pass-rate plummets. In fact you'll get 50% of your powers off at leadership 10 or 108/216 possible dice combinations will result in passing. Now another question arises: since any double 1 or 6 will result in Perils how many of those tests that you passed with also wound your Librarian?

The first step is to look at how many Perils total there are. With 3D6 the total is 32/216 or ~15% of powers will result in Perils. Ouch. Now the convenient thing, for math anyways, is that the Perils are evenly divided between the lower end of rolls and the higher end. This actually means that half of the Perils occur when you pass the psychic test. Neat. So now out of those 108 times your power went off 16 of them will hit your poor Librarian. Which means that in the end only about 42% of the time will you pass your power and not hurt yourself in the process. Ouchies.

Shadow of the Warp is an amazing, under-utilized ability that gives Tyrands a bit of leverage over both Grey Knights and Eldar both of whom employ a lot of psykers in competitive armies.