You don't have to let it end with the big bang, ascension, whatever... some good old epilogue is often highly appreciated (though make sure it fits in the same session as the climactic boss fight/ritual/court session/whatever).
You can use the good old: "What happened thereafter was an even more incredible adventure - but it is another story and will, if at all, not be told today. For other stories await."
Or you can ask your players to each narrate to the rest of the group what their characters are going to do with the rest of their lives as heroes: "I'm going to return to my mountain home, where I will negotiate a lasting peace with the Storm Giants. In the upcoming election for Alderman I am going to stand, and with my heroic reputation, likely win. For the rest of my days, I'll be a good and wise ruler for my people, and war with the Giants will be a thing of the past. My people shall remember me as just, wise, and only occasionally too drunk to stand."
Or you can just provide a fade-out montage: "Imagine the pitted, body-strewn, smoking battlefield on which five heroes with blood on their swords and sparks spewing from burned-out wands gather around the Shadow Demon's carcass. Much will still need to be done: an empire rebuilt, a crown prince brought back from the dead, friends mourned and enemies buried. But for today, there is nothing to do anymore except bind your wounds, and leave this place of death behind. All consequences of your feat are not yet foreseeable, but one thing is clear: you have done a great deed today. Feeling deeply content and proud, but also still strangely numbed by the events of this hour, you lean on one another in your mute procession back to the broken gates of the city. You have reached the end of what you set out to do, and the smokes of the battlefield - or is it the mists of time? - cover the rest."
I even ended a truly epic four-and-a-half year campaign by telling the players that nothing of this had ever happened, and that the PCs would never now know the great story of which they had been a part. That was a time-travelling campaign which revolved around averting a catastrophe, though, so it was what they had wanted. The truly heart-wrenching part was their goodbyes before