Ever had that little niggling voice in the back of your head that tells you to make your players too late to stop the BBEG's final ritual? I know I have. Much like the poster indicates. I also feel that it gives you a chance to practice describing epic pyrotechnics as well. I've yet to pull this on my players. I'm considering it could be a wonderful lead in to a new campaign, depending on their success or failure against whatever it is that the ritual does. After all if you end a campaign with a tpk, you can start the next one 200 years later with the players being part of the revolution against the BBEG. You never know how long demonic forces are going to stick around, and there's always a chance that if you don't keep them appeased they may just drop you entirely from their party invite list. It would be so like the pesky heroes to interrupt the ritual sacrifice and cause your previously friendly demonic allies to turn on you at the most inopportune time.