Sounds like you have no clue what you’re talking about. Because you’re wrong.
The Scourge Invasion was a world event in patch 1.11 that heralded the opening of Naxxramas, the citadel of the dreaded Kel’Thuzad. It was reintroduced into the game in patch 3.0.2 partway through the Zombie Infestation event, in preparation for Wrath of the Lich King, and tuned for the new level cap which players had achieved, occurring after Hallow’s End. It is thus the only non-seasonal world event to have ever been repeated.
There were bosses and zombie disease added to the Wrath event, but they were both similar. And both had quests where you fought Undead and turned in tokens. Not completely different. I played through both. Stop making crap up.