Astragali is a game that was invented thousands of years ago to circumvent luck. Astaragali is usually played with seven hexegonical dice and for djinn is often to test their skills of luck before the more compettitive games of Djinnverso.
When Nimrod first met the twins in the dream, he had them play Astragal. John was able to roll 3 sixes at one time, Philippa was able to roll 4, and together they were able to roll 5. According to Nimrod, this meant they were very lucky together and their powers were stronger. (the chances of rolling 5 sixes at one time is approximately six to the negative fifth power)