Thread:Deathstalker666/@comment-1496755-20170225142933/@comment-1496755-20170401072726

Ah, okay, I see... but do remember that I son't get notifications for edits. So, time for laundry and dusting :P

From what I know, older versions of Doom allowed any boss creature (Baron, Cyber, Spider) to trigger some (don't know if all) of the special actions in levels. Then came Ultimate Doom andm ade it a lot more specific - now E1M8 special actions can ONLY be triggered by killing Barons, E2M8 - Cyberdemons and E3M8 & E4M8 - Spiders. Expectedly, this broke all PWADs that relied upon the old behavior, though luckily ZDoom introduces a compatibility option that fixes this problem, making these maps playable again.

Our twisted rival Wiki explains this pretty good in their article about the Tag 666 mechanic.