I'm thinking of brewing up a Distinction for my group after this upcoming campaign arc - it'd be helpful to take a look and tell me if it's balanced, and if so, at what XP cost (500/750/1000?):

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Peerless Unity (Combat)

All Space Marines, and by extension all Kill-Teams, are trained to operate as a seamless unit, operating flawlessly and in perfect harmony. Some Astartes teams, though, develop this talent of cooperation to even more exceptional levels, moving and fighting as if they were individual components of one mind and one body. Their ability to read each other's body language and unconscious signals ascends to a state bordering on telepathy, allowing them incredible coordination. Once per combat, a Battle-Brother with this Mark of Distinction may, as a Free Action before taking any other actions, exchange his Initiative count with another member of the Kill-Team who also bears this Mark of Distinction and acts lower in the Initiative order. His turn immediately ends, and the selected battle-brother immediately takes their turn at their new Initiative count. When the count reaches the original Battle-brother's new Initiative total, he takes his turn as normal.

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I think I worded it right to patch any double-turn shenanigans, but is the concept sound, and the execution (a Kill-Team who can trade initiative totals amongst themselves for maximum effect) balanced?

I think it's balanced as long as people pay for it Which they will, of course.

The wording sounds OK. Anyway, you're going to use it with your group, so you as a GM will know what the RAI was

As for price...I don't think the bonus is especially overpowered. Of course, it allows you to go into some shenanigans (like letting the Tactical use a squad mode ability that will benefit everyone, letting the Dev mow down baddies before they get the chance to charge, etc), but I'd say somewhere along the line of 500 XP wouldn't sound bad. My own 2 cents, of course.

Well, it's the dream of every GM to write stuff good enough that other people want to use it, so I try to patch RAW holes from the start.

I'll probably put it into a Mark of Distinction package with Brothers In Blood, to get the whole 'We Fight as One' thing going - that'd be 1250 for the lot, I believe, which isn't too harsh.