Overview
The module is 11 chapters, most of them set in different worlds drawn from the history of D&D. A bunch of NPCs from D&D past adventures (not necessarily our adventures) are included which strikes me as an appropriate choice for a 50th anniversary adventure.
The game starts at nominally 10th level. Each chapter grants a level at a milestone which takes the adventure to level 20! That is pretty much amazing for a module. I only know of one other module that reaches level 20 (Dungeon of the Mad Mage) which offered a simple(ish), though very lengthy dungeon crawl.
Vecna is the advertised big bad who must be stopped. That's a classic choice.