Post here when you get an event drop and I'll add it to this running list. When posting please post the ID of the room, room difficulty, enemy, and area. Most of this information is accessible with the /roominfo command. (stolen borrowed from Lemon)
(Some say every ID drops all the event drops, but I'll write them anyway until confirmation)
Ultima Reaper
ALL OF THEM - Indi Belra - Ultimate - Ruins
Chromatic Orb
Whitill - Epsilon - Ultimate - Ep2 - Tower
Skyly - Epsilon - Ultimate - Ep2 - Tower
Viridia - Epsilon - Ultimate - Ep2 - Tower
Skyly - Epsilon - Very Hard - Ep2 - Tower
Bluefull - Epsilon - Very Hard - Ep2 - Tower
Redria - Epsilon - Very Hard - Ep2 - Tower
Banana Cannon
Bluefull - Dorphon Éclair - Ultimate - Ep4 - Crater
Greenill - Dorphon Éclair - Ultimate - Ep4 - Crater
Oran - Dorphon Éclair - Ultimate - Ep4 - Crater
Pinkal - Dorphon Éclair - Ultimate - Ep4 - Crater
Redria - Dorphon Éclair - Ultimate - Ep4 - Crater
Skyly - Dorphon Éclair - Ultimate - Ep4 - Crater
Yellowboze - Dorphon Éclair - Ultimate - Ep4 - Crater
Vampire Cloak (normally from VH Chaos Bringers)
Yellowboze - Chaos Bringer - Very Hard - Ep1 - Ruins
Viridia - Chaos Bringer - Very Hard - Ep1 - Ruins
Bluefull - Chaos Bringer - Very Hard - Ep1 - Ruins
Redria - Chaos Bringer - Very Hard - Ep1 - Ruins
Skyly - Chaos Bringer - Very Hard - Ep1 - Ruins
Purplenum - Chaos Bringer - Very Hard - Ep1 - Ruins
Girasole (normally from VH and Ult Shambertin)
Greenill - Shambertin - Ultimate - Ep4 - Big Pit
Redria - Shambertin - Ultimate - Ep4 - Big Pit
Purplenum - Shambertin - Ultimate - Ep4 - Big Pit
Whitill - Shambertin - Ultimate - Ep4 - Big Pit
Whitill - Shambertin - Very Hard - Ep4 - Big Pit
Halo Rappy Soul (Jack o Lanterns)
ALL OF THEM - Halo Rappy - All difficulties - Ep2 - Temple
Last year's list