Cross-posted from the Deathwatch forums since it involves mixing both games.

In my campaign we have a group of five DW Marines and one female player playing an Inquisitor/Psyker transplanted in from the Dark Heresy setting.

In DW Genestealers have the Trait "Tyranid" in the Dark Heresy setting they have the Trait "From Beyond". These two traits are essentially the same; just different names for the same benefits. What ever it is called the trait states, in part: "It is immune to Fear, Pinning, Insanity Points and Psychic Powers used to cloud, control or delude its mind." Preceding taken from page 330 of the Dark Heresy Book.

In your opinion would creatures with this Trait be immune to the Telepathic Psychic Power "Psychic Shriek" found on page 180 of the Dark Heresy book?

I would say "no immunity". It could be said that psychic shriek is asserting a type of control over the mind by overloading the senses, but I think that sounds far fetched. I would argue that psychic shriek is akin to a brute force attack against a creature's mind and not an attempt to exert control (like dominate or terrify).

I would rule that psychic shriek overloads the tyranids connection to the hive mind and that no immunity applies.

I'd say that a tyranid creature that is within their psychic network, the synapse, would be immune. So for example a genestealer nearby a hivetyrant or a genestealer magus wouldn't care, since the collective hivemind is too powerful to care about a silly little human. However, a stealer or even a group of stealers on their own would certainly be affected by the power. Since it is more of a brutal force attack than any kind of detail manipulation.

Psychic shriek seems to be more in the 'destroying' rather than the 'controlling, deluding or clouding' business when it comes to minds.

The group's Pskyer is a 9th level Ascended character with a WP bonus of 6. This means that Genestealers (with 60 Toughness) take only two hits to neutralize with this power. I know that once the character gets 500 XP (giving her 1000 unspent) that she will buy the Unnatural WP talent. This will mean that Genestealers will become a minor annoyance rather than a real danger. Of course there is always the chance of a successful ambush but the five Deathwatch Space Marine players are always careful to keep the Inquisitor well guarded.

I think that I will go with the suggestion that this ability works as normal against genestealers that are not in the company of a synapse creature. When, and if, the group hunts down the Broodlord the genestealers in its company will be protected from Psychic Shriek by the shadow of the Hive Mind.

Oops! I just found out about an error I made with the Psychic Shriek power. Any creature in the area of effect is entitled to a WP Test to avoid the effects. That makes it much less of a threat to play balance.