On the subject of the "Apocalypse Adventures":
Near the end of 2E, WotC began promoting a set of adventures purportedly designed with the sole purpose of destroying your game world in mind. In actuality, what was understood (but never said) was that these adventures would be what you used to end Second Edition and begin Third Edition in your campaign.
The Apocalypse Stone was meant for any homebrew worlds. It was careful to be vague in referring to gods, and the locations were easy to insert as being somewhere remote and unexplored. There were some connections to other products (for example, talking about Grand Duke Moloch and the Reckoning of Hell, from Guide to Hell), but by-and-large it could be used in any non-specific world. Looking at the end of the product, it's quite obvious that it's laying the groundwork for 3E, since it talks about how the aftershocks of the adventure being the cause of half-orcs, monks, barbarians, and sorcerers all becoming prevalent.
Die Vecna Die was the flip-side to The Apocalypse Stone. Whereas The Apocalypse Stone was meant to be used for a generic campaign workd, Die Vecna Die was meant to be used in the holistic 2E multiverse. The adventure began on Oerth (the World of Greyhawk), went to Ravenloft, and then ended in Sigil (the main setting for the Planescape campaign). The end of the adventure ends in all reality (in all space and time) being restructed, hence why the current campaigns (Forgotten Realms, Greyhawk, etc.) are now in 3E and not 2E. In effect, Die Vecna Die is the reason everything the multiverse switched editions.
It's worth noting that some advertizements listed there as being three Apocalypse Adventures. The Apocalypse Stone and Die Vecna Die were usually displayed the most prominently, but in smaller print there was a mention of Reverse Dungeon, and replacing that later on was The Dungeon of Death. Neither of these actually did anything "apocalyptic" to your campaign however. Reverse Dungeon was something of a set of adventures (almost a mini-campaign) where you played monsters on various levels of a dungeon (goblins on the first level, classic "aberration-style" D&D monsters on the second level (beholder, mind flayer, doppelganger, etc.), and undead and demons on the third level) defending your abode from invading adventurers. The Dungeon of Death was a Forgotten Realms adventure, a dungeon-crawl through the dungeon of the same name.