This time it’s a request from the gaming club The Warheads and we are going to cover Psychic powers, or more specifically* things that stop them*. So let’s set up a few scenarios.

A sinister Slaaneshi Daemon Prince tries to castLash of Submissionon a squad of Space Wolves near aRune Priest, if the Prince tried the same on a squad of Grey Knights near a Dreadnought withReinforced Aegis, would he be more or less likely to succeed?

There are a **wide variety of psychic defenses** out there, and they use different game mechanics to achieve the same objective i.e. to stop the psychic power. To successfully use a power the psyker must roll less than or equal to his leadership on 2d6. Most psykers have a high Ld and this isn’t something you’d generally expect to fail very often. The mechanics used to stop them fall into **two types**:

**Type 1)** effects that make it harder to pass the psychic test e.g. reducing the Ld value of the Psyker (Aegis, Reinforced Aegis) or increasing the number of dice rolled for the test (Runes of Warding, Shadow of the Warp)

**Type 2)** effects that happen after the psychic test to try to block the power, e.g. the leadership roll-off for a psychic hood, or the 4+ nullify from a Rune priest.

**Spacewolves** can have a special ‘layered approach’ where they get a 5+ nullify from a Wolf Tail Talisman, and if that fails they can then try the 4+ from the Rune Priest. If the Rune Priest happens to be **Njal Stormcaller** then that 4+ becomes a 3+. But even with all that wolf-cheese, the master of psychic defense has to be **Khorne**.

The** ‘Blessing of the Blood God’** gives a 2+ invulnerable save against any wounds from psychic powers, and his most beloved champion **Kharn** is flat-out immune to psychic powers. Though to be fair these abilities are not widespread, and the Chaos Marine codex has pretty much no psychic defense. *(Update: it’s been pointed out by a few people that Lady Malys of the Dark Eldar is also immune to psychic powers, as is any unit she joins – so it’s not just Khorne who can totally nullify psykers!)*

The effectiveness of** Psychic Hoods** varies depending on the relative leadership values of the defender versus the caster. If the caster has a low Ld compared to the defender then he’ll have a very tough time.

So for all these **‘Type 2’ defenses** I’ve simply calculated the odds of blocking the psychic power (assuming that the psychic test is passed). For the hood I’ve calculated it for all possible differences of leadership value between caster and defender. I’m sure the range encountered in practice is smaller, but I did it simply for completeness.

The table is ordered from **best defense to worst defense** as follows:

Psychic defense |
Chance of Nullify |

Psychic Hood with Caster Ld 6+ less than Defender | 100% |

Psychic Hood with Caster Ld 5 less than Defender | 97% |

Psychic Hood with Caster Ld 4 less than Defender | 92% |

Psychic Hood with Caster Ld 3 less than Defender | 83% |

Blessing of the Blood God | 83% |

Wolf Tail Talisman AND Njal (5+, 3+) | 78% |

Psychic Hood with Caster Ld 2 less than Defender | 72% |

Njal (Straight 3+) | 67% |

Wolf Tail Talisman AND Rune Priest (5+, 4+) | 67% |

Psychic Hood with Caster Ld 1 less than Defender | 58% |

Rune Priest (Straight 4+) | 50% |

Psychic Hood with Caster Ld = Defender | 42% |

Wolf Tail Talisman (Straight 5+) | 33% |

Psychic Hood with Caster Ld 1 more than Defender | 28% |

Psychic Hood with Caster Ld 2 more than Defender | 17% |

Psychic Hood with Caster Ld 3 more than Defender | 8% |

Psychic Hood with Caster Ld 4 more than Defender | 3% |

Psychic Hood with Caster Ld 5+ more than Defender | 0% |

Ok so that’s Type 2 defenses, **what about those Type 1s**?

I start by graphing how the **odds of passing the psychic test vary with increasing leadership**, for a variety of scenarios (including Runes of Witnessing which are not a defense at all, they are there to improve a casters chances of passing).

So we get a set of S curves, where the red curve represents a ‘normal psychic test’ and any curve below it is hampered by a defense. **The lower the curve in the graph, the stronger the defense**. As you can see the Reinforced Aegis is the meanest of the lot. It is worth noting that the **Runes of Warding** give a perils result on any result of 12 or above, and increases the odds of a double 1 to push the odds of **perils from about 5.6% up to nearly 45%**, that is a serious threat in its own right.

While this clearly shows the effect of the defenses, it can be a bit tricky to read the **actual impact** of the defense. So with that in mind I’ve also plotted the difference each defense makes for a given Ld value, shown below.

This shows **specifically** what these defenses are worth against a given Ld. The Normal Psychic test is a flat red line as it compares exactly to itself (if you get what I mean). Runes of Witnessing are above the line as they *improve* the odds of casting, and the defenses are below the line because they reduce the odds of passing.

**So what about that original question**? Which target is easier for the Daemon Prince. Well he’s Ld10 so under** normal circumstances he has a 92% chance of passing** his psychic test, the Rune priest has a **50% chance of blocking** him if he’s successful giving an **overall probability of 46% for making the Lash**. With the **Reinforced Aegis**, the odds of making the psychic test drop to **42%** making it slightly **more effective** than the Rune Priest.

I’m sure there are lots of particular scenarios you could come up with, and hopefully I’ve given you the data to be able to make like for like comparisons, but **don’t be afraid to put a request in for a specific situation** if you can’t work it out!