Inspired by several scenes in the Eisenhorn novels and by the new Codex Imperial Guard Psyker Battle Squad...

Psychic Choir (Talent, Prerequisite Psy Rating 1, cost 200 exp)

Characters with this talent may form a psychic choir. One psyker is chosen as the leader of the choir, with the other psykers providing a portion of their power to strengthen the leader. A choir may contain a maximum number of psykers equal to the choir leaders Willpower bonus. For each member of the choir, the choir leader gains a +1 bonus to his Power Roll. If the choir leader invokes a Psychic Phenomena all members of the choir are affected.

I'm not entirely sure that captures what I'm looking for, but consider it a springboard for discussion.

I'd consider it a little weak - you have to get a semi-expensive talent for every psyker character and you won't be getting more than about 5 points out of it (should you happen to have five psykers lying around somewhere). Further, the individual strength of the psykers doesn't matter anymore. How about the following:

Psychic Choir (200 XP, Psy Rating 1):
You can join an existing psychic Choir and lend your power to it. The Choir has its own psy rating, calculated in the following way: Square the psy ratings of every member, add them, then square root the result. Any phenomena produced by the Choir affects all its members. Linking to a Psychic Choir takes a full action and is automatically successful if the Choir's Cantor accepts the link and you are within 10 metres of the Cantor. Withdrawing from a Psychic Choir is a half action that is automatically successful. While joined to a Psychic Choir, you cannot manifest powers on your own. Should you end a turn farther away than 10 metres from the Cantor, your link to the Choir is severed and you lose 1D10-2 wounds, reduced by your willpower bonus.

Psychic Cantor (200 XP, Psy Rating 3, Psychic Choir):
You can form a psychic choir and manifest powers you know with its psy rating. Alternatively, you can also join another Cantor's choir and still use it to manifest powers after you take over the leadership of the choir. Establishing a psychic choir takes three full actions and a successful challenging invocation roll. Taking over the leadership of another choir when you have already joined it takes a full action and a successful routine (+20) invocation roll. Should you fail this roll by 3 or more DoF, the choir dissolves and everyone involved with it loses 1D10-2 wounds, reduced by their willpower bonus. You cannot take over the Cantor post unless the active Cantor allows you to.

Warding Choir (100 XP, Psy Rating 3, Psychic Choir):
When you join a psychic choir, you can choose to split your psy rating between the Choir's psy rating and the warding pool. The warding pool is calculated the same way as the psy rating. For every two points in the warding pool, you can designate one D10 that is rolled seperately from the rest and cannot produce phenomena when the Cantor manifests powers from the pool.

Long Distance Chant (100 XP, Psy Rating 4, Psychic Choir):
You can join and maintain contact to a Psychic Choir over a distance of your willpower characteristic in metres.

Psychic Dissonance (300 XP, Psy Rating 6, Psychic Cantor):
You can force your way into a psychic choir, take over its Cantorship and even prevent others from leaving. To join a choir against the will of its Cantor, use a full action and make an opposed invocation test. To take over the Cantor post, spend a full action and make an opposed invocation test with you getting a -20 penalty.
You also have a number of further options while you are the Choir's Cantor: Should you want to prevent another Choir member from leaving, make an opposed invocation test with a -30 penalty for the Cantor. You can also collapse the Choir as a full action. This deals 1D10-2 damage to you and 1D10+5 to every other member (reduced by WP bonus). Severing the link by moving farther away than 10 metres deals an automatic 1D10+2 damage to the leaving member should you so wish.

Psychic Library (300 XP, Psy Rating 6, Psychic Cantor):
While acting as a Choir's Cantor, you can use every power that is known to any member of the Choir. If you use a power that is not known to you, but you possess a power of the same discipline (or if it is a minor power), the threshold is increased by 3. If you use a power from a discipline not known to you, its threshold is increased by 6.

Any invocation rolls of these talents are granted cumulative +10 bonuses for possession of a Psy Focus or the skill Perfom (Sing).

Interesting idea, but I'd like an easier way to calculate the choir's strength. I tend to think that any rule that requires me to use calculator is a bad rule.

It is easy - you should be able to square numbers from 1 to 6. Adding shouldn't present a problem either and the square root can simply be guessed by looking at the squares of individual numbers since you don't need the exact root but only the next-lowest integer.

Say you have two psykers of level 4 and one of level 5. This gives us a psy rating of SQRT(4*4+4*4+5*5) = SQRT(16+16+25) = SQRT (57). 6*6 is 36, 7*7 is 49, 8*8 is 64 - so you now have a psy rating of 7.

I chose it because it's a comparably simple way of creating a non-linear function that both always gives a number that is higher (assuming at least two participating psykers) than the strongest individual psyker but is not excessively strong so you don't make high-ranking psykers obsolete. If you have a different function that fulfills these objectives, I'd be interested.

Further, it's not exactly a common event for several psy 4+ psykers to link up unexpectedly, is it?

Easy

I have to say the math may be “easy” but that is way too cumbersome to be efficient.

The idea of a choir should be to lessen the risk of activation and add to the overall power.

I think the better way to approach the problem would be to gain a bonus equal to ½ the WP bonus of each Psyker in the choir to all rolls.

This way of doing it would lead to a more powerful choir but it would be easy to calculate.

The trade off should be that the choir should take 2 full rounds to join and once you have joined a choir the individual can not leave the choir voluntarily. Only the Cantor has the ability to dismiss a choir member. If the Cantor is killed then all choir members are stunned for 1d5 rounds and must make a challenging or tougher WP test or take damage equal to the Cantors WP bonus.

Then you add a trait that allows you to leave the choir of your own accord or maybe one that shields you from the choir.

The way I was going to be doing it was to have the characters who want to join into the psychic choir all spend 100xp for the talent. In the Imperial Guard book it makes the choir seem rather permanent, and not to be taken lightly, so I wouldn't really allow them to just willy-nilly join or escape it. They could do so, but rejoining this or that one would be another 100xp drop.

Psychic Choir: The psyker has joined a psychic choir, and has mastered the ability to meld their particular warp signals with that of their fellows.

Compose another "character" as Sanctioned Psyker career, but with only one attribute, and no skills.

This character serves as the choir. Their Willpower, one of the two stats, equals the median of all the psykers joined with the WP bonus of all those with a WP above this added in. Example: Willpower 30,30,30,35,40,40,45 would mean a Willpower 35 Choir character, with an additional +4+4+4,+12, equalling 47. In a way this discounts low level characters, and they can actually weaken the choir ( which would make sense, to a degree, like having a bad flautist in an orchestra ) but it is a sorta simple system.

The Psi-rating is devised as the lowest Psi-rating, plus one for every Psyker in the squad. Psykers with a Psi-rating at least twice that of the "Base Psi-rating" count twice, that way really powerful psykers in weak groups still give a tangible benefit, and this can technically add up to a really powerful effect. I am questioning whether or not to limit this by the highest Psi-rating multiplied by the lowest, or something along that.

Lastly, any Psyker in the choir can access the "Choir" character, who has access to all of their powers. However, any time that someone performs a Psychic Power and wants to use the Choir stats -all- of the other members must spend their reaction to, basically, sing along. They can all reflexively feel that one of them wants to do this, and for every one that does not ( or can not ) accompany you subtract their Psi-rating from the Psi-rating of the choir, and their Willpower bonus from the Willpower of the Choir ( They throw the choir out of harmony, and it makes a big effect. I figure this alone should discourage really large battle squads, by the merit. ) This counts as the casting Psyker and the Choir's Psychic Power action for the round, but only as a use of a reaction for the rest of the choir.

Its a little munchy, but once you write everything down it should play well in the game. Thoughts?

The Psi-rating is devised as the lowest Psi-rating, plus one for every Psyker in the squad. Psykers with a Psi-rating at least twice that of the "Base Psi-rating" count twice, that way really powerful psykers in weak groups still give a tangible benefit, and this can technically add up to a really powerful effect. I am questioning whether or not to limit this by the highest Psi-rating multiplied by the lowest, or something along that.

You should definitely put some limiter in that one. Compare a choir with members of the levels 1,2,2,2,2 and one with the members 2,2,2,2,2. The first one has a power rating of 9, the second one a rating of 6.

Good idea.

For simplicity.... lets just say max Psi-rating of highest Psi-rating, plus 1 Psi-rating per character in the choir ( including the Psyker. ) This makes it less important for high level Psykers, but makes it very very nice for lower level... which is what Psyker Battle Squads are anyway.

Now, the math. I re-read my post and it looks long, so I'm going to simplify it.

WP: Median Willpower, plus WP bonus of all members with a higher Willpower.

Psi-rating: Lowest Psi-rating, plus one for every member.

Powers: One member can activate the Choir per round, with every other member using a Reaction to assist. For every member who does not, or cannot, lower the Psi-rating by their Psi-rating, and the Willpower by their Willpower Bonus. This counts as using a power for the Choir, and the one who uses it, but only as a reaction for all other members ( who can use one less dice in their Power Pool until their next turn. )

I added that last part, too, because Psykers are already very powerful. Having a choir of 1,3,3,3 would be an awesome force, and that helps to balance it a little bit.

AH ha ha haaaa. Game mechanics have to be based on the 5 beer method for our group. There is no way in hell I can take the square root of anything after 5 beers. Even figuring out damage can be a trick when sloshed at 1 AM.

Our Psychic choir rules are very quick and dirty. Our psykers can simply add their psychic dice together OR if any psyker does not use a power die they can nullify one rolled 9 for any die they don't use. Basically they are absorbing the power overload.

This way the psykers can pull off some really powerful skills when they need them OR they can practically guarantee safely casting easy skills.

Quick, easy, effective.

Also we have come up with a couple of really powerful spells that require a lot of energy, that one person would have a very hard time casting.