In Expert Mode, Professional Mode, and Slayer Mode, bosses drop Treasure Bags which contain special Expert/ Professional/ Slayer-exclusive items in addition to their normal drops. Beating all of the bosses in Expert, Professional, and Slayer requires alot of determination, and time. After the player has beaten the Moon Lord, a whole new level of progression is unlocked.
Any boss monster generally has much stronger attacks than a normal enemy, more defense, and considerably higher HP ranging from 750 to 25,000,000, 1,250 to 40,000,000 in Expert Mode, 2,500 to 56,000,000 in Professional Mode, 4,000 to 65,000,000 in Slayer Mode, and 24,000 to 80,000,000 when the Possessed Heart of Oblivion is activated. This is a list of bosses that are introduced in the Spectra Mod.