Master used the contract stone on the eldritch beholder when he visited Mistress in Aurebor. This allowed him to gain an additional eternal-grade combatant for a day. The contract stone had different grades. To create a clone of an eternal-grade native, a legendary grade was needed. That was what Master possessed.
A contract stone couldn't be used if the native it was contracted to was nearby. Master prepared this in case the eldritch beholder was killed or decided not to come to Liguritudum after Aurebor lost. He needed the eldritch beholder to utilize her brood.
When the clone of the eldritch beholder appeared, the woods on the Western side of Dritzuut came to life. Within these woods was a cave. Inside this cave were the thousand eggs the eldritch beholder laid during her short visit to Liguritudum. Several mature beholders were tending these eggs.