I'm not going to spend a lot of time on it but I wasn't a fan of the plot of WOTC's Vecna – Eve of Ruin adventure. Without spoiling, I'll say that the adventure hinges on one bit of deception that, when revealed, unravels the whole purpose of the rest of the adventure. That's not great.
But fear not! We can build our own Vecna adventure!
This article contains a campaign outline you can use or to inspire your own Vecna-based multiverse-spanning adventure. Can you refactor material from Vecna Eve of Ruin into something good? Probably. You know what else you can do? Build your own exactly the way you want it and save yourself $60.
Let us begin.
As a mortal-become-god, Vecna continually sought the one thing he wanted most – power. Until, at the culmination of his might, he realized his folly. In his quest for power, he lost his happiest moment forever – a moment of true happiness side-by-side with his partner, Kas, before the two of them began their quest for power. This drive for power destroyed them, sending Kas to the domain of Dread known as Torvag after his betrayal of Vecna an Vecna on his insatiable quest for godhood.
Realizing his loss and recognizing he can never find that happiness again, Vecna seeks to undo reality. He wants to roll back the multiverse to the moment before he and Kas began their terrible path. In doing so, he will destroy everything and everyone in all worlds that came after that one moment.
Thus, Vecna travels to Pandemonium to conduct the ritual of unmaking and its up to our heroes to stop him.
Meanwhile, Kas believes he has escaped his prison world of Torvag, though actually released by the dark powers holding him there. Along with his fanatical followers, Kas plots revenge against Vecna. He seeks the rod of seven parts, spread across several worlds of the multiverse: Oerth, Athas, Krynn, Toril, Eberron, Barovia, and the Astral Sea. Kas knows only the rod, and the entity it releases, can give him the power to defeat Vecna. With the rod in his possession, he can call forth one of the most powerful horrors in the multiverse, Miska the Wolf Spider.
Kas's cult of vengeance spreads out to these worlds seeking the pieces of the Rod of Seven Parts.
Our heroes begin by infiltrating a forgotten temple deep beneath Neverwinter where they face a powerful cult of worshippers of Vecna, including a lich in his service. On the lich's defeat, the characters discover that Vecna has begun a ritual of unmaking in the plane of pandemonium. They are contacted by three wizards, Mordenkainen, Alustriel, and Tasha.
The wizards three know the only way to defeat Vecna is to gather the rod of seven parts which can pierce through his divinity and bring him down to his original archlich self. They are currently unaware that Kas too seeks the pieces of the rod. The wizards aid the characters by discovering locations where the pieces might be found, teleport the characters to those locations, and to attempt to contact the gods for aid (which is unsuccessful – the gods simply don't believe them.)
The wizards offer three locations the characters can choose from where the wizards know pieces of the rod might be kept. When the characters reach the second location, they run into the Kas's Cult of Vengeance also seeking the pieces of the rod. The piece at the third location, the wizards discover, has already been captured by Kas's cultists. The wizards then offer the next three locations.
After the characters return from their fourth world, the cult of Kas now has recovered two pieces from two other worlds, leaving a final world in play. At this final location the characters face the strongest followers of Kas and attempt to recover a fifth piece.
With their own pieces in possession and knowing that Kas has the remaining pieces, they travel to the prison realm of Miska the Wolf Spider. There they face Kas and Miska and, should they successfully defeat the two forces, receive all seven parts of the rod.
With all seven pieces in hand, the characters assemble the rod and face Vecna himself in the center of Pandemonium. Their use of the rod makes the elder god mortal once again, though extremely powerful. If the characters succeed, the multiverse is saved. If they fail, all reality is unmade to a time thousands of years previous – the last happy moment for the archlich.