I knew the answer was right there in front of me.
Now you have to live the rest of your life knowing you're going to die someday and disappear forever.
The Cataclyst was a magic-based bomb that, when activated, destroyed everything in the current sector of the OASIS; every OASIS avatar, NPC, building, and object would be destroyed and would remain so until the server reset at 12:00 PM OST. The name is a portmanteau of "catalyst" and "cataclysm".
The Cataclyst was used by IOI following their defeat at the hands of the surviving four members of the High Five and over half the population of OASIS. Detonating it, they killed off what remained of their army, Castle Anorak, and half the avatar population of the OASIS. Parzival was revived after the detonation of the Cataclyst with his 1981 Quarter artifact.
The Cataclyst had been sold in auction and, until the sale of Fyndoro's Tablet of Finding, was the highest selling item in the history of OASIS.
Gunters would sometimes state they had the Cataclyst and threaten to detonate it when they were surrounded by Avatars they didn't like.
When activated, the Cataclyst would destroy every living and non-living thing in the current sector, including the user. Any landscaping, including trees, hills, mountains, and rivers, would be destroyed and replaced with a grassy field until the server reset.
The magic-based nature of the artifact, however, indicates that it cannot be used in tech zones or null zones (where magic based items do not work)
In the film, the Cataclyst first appears when Parzival and Aech visit Avatar Outfitters for Parzival to use his newly-won fortune to buy several upgrades. The Cataclyst is on sale for two million credits and Parzival explains to a horrified Aech that it zeros out any avatar on a planet, including the user. When asked who would want such an artifact, Parzival points out that IOI would.
During the final battle, the Cataclyst is produced by I-R0k for Nolan Sorrento who threatens to detonate it if Parzival doesn't surrender. I-R0k is horrified as he'd thought it to be a bluff and attempts to escape chased by Sho as Parzival and Sorrento fight over the artifact. Sorrento manages to detonate the Cataclyst despite Parizval's best efforts, zeroing out himself and every other avatar on Planet Doom, gunters and Sixers alike. However, the Extra Life granted to Parzival by The Curator enables Parzival to survive, albeit without any of his coins, items or weapons.