Win the Halloween boss fight, which has Pumpkin Brute and Pumpkin King phases. Check your Boss Mastery counter in the Halloween Dimension and claim the reward when available. The interface states “1 Boss Kill = 1 Mastery” and shows Locked / Claimed states; do not assume each phase independently adds a point or that rewards are delivered automatically.
These are cumulative milestones, not numbers to add together, drop percentages, Candy Corn prices or pet levels. For example, Coffin Crawler needs 10 total kills, and Pumpkin Brute needs 65 total kills.
The Pumpkin Brute Eternal pet at 65 kills is distinct from the boss fought in the arena. Pumpkin King Divine is a separately announced chance, with no confirmed percentage; Cursed Monarch comes from fusion. Neither belongs to these milestone rewards.