With the elemental guardian disposed of according to plan, Kain's focus shifted immediately to the Prismarin. If it wasn't for this new contract, then the same way he'd meticulously planned to get rid of the elemental guardian, he could have removed the remaining Starweaver and beaten Serena for the first time.
With the dozens of Starweavers it had produced, it was impossible for Kain to accurately determine which one was the true body—at least not until the true body unleashed a long-distance attack.
To counter this, Bea blanketed the field with threads. They couldn't control the illusions themselves, obviously; instead, they acted as tripwires, alerting Kain to any solid movements in the chaotic field of illusions.