The Tablets of Fate were the lists of every god that existed in Faerun, and then detailed their duties, their portfolios, and so on. Bane and Myrkul stole them from Ao, and as punishment for this (and for all the gods in general paying more attention to their own squabbles with each other than to their mortal followers), Ao forced them to choose one avatar, and cast them down into the Realms in that form. The only one to escape this fate was Helm, who was charged with guarding the gateways into the deities' planes of existence. As a matter of fact, I think that's how Mystra was killed -- she tried to return, and Helm cut her down.
In the seconds before her last breath, she dumped the essence of The Weave into a number of mortals, and Elminster hosted the brunt of it. In doing so, he lost all of his spellcasting abilities, so he had little more than a few magic trinkets and his fighter and thief levels to preserve him, along with the Rangers Three (this is all in the Shadows of the Avatar trilogy, which you are now hereby bound to read). Anyway, this is when all the magic went nuts. Since Drizzt and the Friends of Mithral Hall are so far removed from civilization during most of this, Salvatore only touches on it briefly.
In the meantime, the rest of the pantheon began realizing that while they're mortal, their foes are, too, and so many of them set out to settle old scores once and for all, and thus you have the deaths of many gods during this time. Opportunistic Cyric also realizes this, and he does his own share of deity-slaying, with his sword
Godsbane -- the avatar form Mask had chosen when he was cast into the Realms, unbeknownst to anyone but himself. The downside is that he takes a *big* hit in power when the sword is destroyed. But anyway, this is why all the gods in the Realms hate Cyric. I think he's the only one listed in the FRCS without any allies.
Once the mortal wizard Midnight, along with the mortal Kelemvor, tracked down the Tablets of Fate, the Time of Troubles ended and Ao set things as he saw fit, returning the remaining deities to their divine forms and bringing Torm back to life (since he died in service to his ethos and thus demonstrated to Ao that his godly duties were important to him). Midnight ascended to become the new Mystra, taking back The Weave into herself, though I don't think Kelemvor ascended at this time.
And I just realized that this is all available on page 264 of the FRCS.
