Igor needed Hustace to provide him with details on how to unleash Lightning.
This was something Igor wasn't as clear about as Hustace. Igor only knew that his stone contained the power of Lightning, and now he needed that power.
Using the power of Lightning to launch a final attack on the Puppet Skull was part of Igor's last plan, and the most crucial part.
Hustace naturally shared all his knowledge about Lightning with Igor without reservation. Strangely enough, while Igor had been crucial in creating the imitation Rain Crystal Stone—the most critical step being implanting the Lightning into the stone—he didn't understand how to unleash it, even though he knew how to place it there.
This was truly a baffling matter. Fortunately, with Hustace's help, Igor was able to continue executing his plan.
