Hey kid, over here. You wanna do some math?

Ok folks, time for some math-hammer

I'm going to go over some of the dreaded power-lists that rely on getting combinations of psychic powers or abilities, and work out just how likely it is to get them all to work

First off, and old favourite: Psychic Shriek & Weaken Resolve.

The idea behind this is to use a Imperial Guard Psyker Battle Squad to reduce the leadership of a unit to as low as possible. Then make them take a leadership test on 3D6, suffering wounds equal to how much they fail by, with no armour or cover saves. It's a pretty nasty spell, able to wipe out entire squads of heavy infantry and tough creatures, only good psychic or invulnerable saves can protect you.

To demonstrate how totally OP this is, lets try and kill a small squad of mediocre troops; 5 Chosen Chaos Marines, at Ld10. 

After that, we'll try and cripple the heart of a tyranid army, killing off a Tervigon.

The steps involved are:

1. Cast Weaken Resolve
2. Cast Psychic Shriek
3. Roll to Hit with psychic shriek
4. Kill 5 or more rebel scum

Step 1:

The power is cast on Ld9, so that's a (30/36) chance of passing. Pretty good so far.

But then there's a Deny the Witch roll, but that's only a on a 6 right? Who 6's anyway...

So are running totol is now (30/36)*(5/6)

Step 2:

Psychic Shriek will generally be on Ld10, (33/36). Then there is a another Deny the Witch, so (33/36)*(5/6)

The total so far is (30/36)*(5/6)*(33/36)*(5/6), which is only about 53%. Ugh oh...

Step 3:

Rolling to hit. Most imperial Psykers are Bs4, some of the more exotic are Bs5 or higher. Let's assume it's an imperial psyker, as the are Imperial Guard around after all...

Bs4 has a (2/3) chance to hit.

Step 5:

Finally the chaos insurgents have to actually take some casualties. When reduced to Ld2, taking 5 or more casualties means rolling a 7 or higher on 3d6. Which is (196/216), close to 90%

The total success rate is (30/36)*(5/6)*(33/36)*(5/6)*(2/3)*(196/216); 

32.1% success rate. That seems pretty crap. Over a 6 turn game, this should only work twice.

Maybe a nice gimmick to wipe out a key unit at an opportune time, but not all that reliable.

To make matters worse, if the enemy has a better deny the witch roll (5+ instead of 6+), chances go down to 20.5%

Against Grey Knights, with -1 Ld to cast psychic powers against them and a 5+ deny the witch, chances drop to 16.2%

Going back to the example of killing off a Tyranid Tervigon with 6 wounds:

The additional defenses are Shadows in the Warp (-3 to Ld), and a 5+ deny the witch. Chances to deal 6 wounds in a single shot are a mighty 6%

Totally OP. GW need to fix this right now. Or like, whenever.