I'd be tempted to think of the 'broos' version as the colloquial equivalent of 'youse' instead of 'you'. Or more plausibly, a very archaic version of the plural, as the Lightbringer's Summons is undoubtedly phrased in an ancient and ritualistic way. I believe that older versions of a language are frequently more grammatically consistent than usage will make them over time.
Alison
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