Rules might

Kaiser and Alex remained at a suitable distance from the battlefield, far enough away to avoid being caught in the Sin of the Forest's attacks while still being able to observe the fight clearly.

Most of Kaiser's undead army had already scattered throughout the forest, but his strongest undead remained near the Sin of the Forest.

Selene, Rachel, and the Corpse Lord surrounded the monster from different directions. Instead of immediately trying to kill it, they focused on defending themselves and carefully observing the power of its Rule as he attacked them with his corruption rule.

They were all naturally aligned with different aspects of the power they possessed, and fighting against a Rule wielder could provide them with valuable insight. If they could experience the Greater Rule of Corruption directly, they might be able to better understand how Rules functioned and eventually develop their own comprehension.