The history of Dungeons & Dragons as a franchise is a little confusing. Once called Advanced Dungeons & Dragons (AD&D), that’s not the case anymore. What happened? As ScreenRant explains, the transition happened with help from Vecna.
Old-school D&D is, well … hard. There are a ton of rules, even more restrictions, and THAC0 was a thing. Long story short, when AD&D Second Edition came out, it was produced by TSR. TSR would change hands and ultimately face bankruptcy before being bought by Wizards of the Coast, back in 1997. WotC gave D&D a complete overhaul to make it much friendlier to new players, and it included major changes to rules, a streamlining of play, a lowering of the learning curve, and in-game changes like the addition of a sorcerer. (That’s a class that has innate spellcasting abilities, as opposed to a wizard that needed to study to learn magic.) Combat became much more tactical, most things could be settled with the roll of a d20, there were no longer limits on class-race combinations, and characters could advance in prestige beyond level 20 (via DiceBreaker). They didn’t just erase everything prior to the new 3e, though. Instead, they looked to Vecna.
Thanks to Vecna’s invasion into the City of Sigil, the shockwaves the fight sent through the multiverse, and the Lady of Pain’s attempts at holding all of existence together, the changes between AD&D to D&D were actually canon. All those rules changes? Class changes? They were what happened after the Lady of Pain put the world back together following Vecna’s attempt to break it.