Phase 1: Transparent Boss
During its initial phase, the Plague Beast is transparent and invulnerable. It will slowly fly after you until you have touched 8 (?) different Plague Hives scattered around the planet, forcing it to enter its second phase.
Phase 2: Visible Boss
During this phase, the Plague Beast is completely visible and takes damage from all attacks. It will repeatedly dash in straight lines until death.
Challenge Level Effects
The Plague Beast gains additional health and damage on Challenge Level 1 and above, but has the same appearance and behavior as before.