I would call it "The Ruined Realms".
As there are an infinite amount of Prime Material Worlds (ala Toril, Greyhawk, etc), this is easily enough done.
What you definitely need to do is establish a REASON that all the Gods would unite together.
I would suggest the Tharizdun idea, myself. Say that Mankind had freed him from his imprisonment (make it up! Or even better, let your Players do it...hehe. Great way to start a new campaign from an old one IMHO).
The Gods have to band together to re-imprision/defeat Tharizdun (which causes unbelievable damage to both Toril and the Planes) and the surviving Gods decide to punish the Prime Material for the deed.
https://en.wikipedia.../wiki/Tharizdun
And...I will make this up :
The Gods, in a desperate last bid to contain Tharizdun, bound him to Toril itself, which caused massive disruption planetwide. Continents were broken, seas sundered, and calamities of all sorts broke out. Populations were reduced to less than 1%, and survival became the only important thing to those still alive.
Due to the imprisoning of Tharizdun, the Gods then retreated from Toril, leaving it's inhabitants to their fate. The ways to the Crystal Sphere itself were closed, and it became a dead, isolated place...alone, without gods or means of escape. Only Tharizdun remained, imprisoned, but alive...