It's found in the nature of a man, most likely
Exactly. Curses, now you've done it. Why? Because I can imagine how "tribal and primitive race of lizardmen conspires to reclaim their primal powers from the forgotten past when they ruled the world as gods" could be a pretty nice adventure hook in the hands of a competent DM, or a nice story hook in the hands of a blasphemous writer.
Of course, you'd have to drop the bullshit about lizardmen being evil - they are merely non-human, alien and incomprehensible - good and evil are delusions of weak mammal moist robot manboon brains, not shared by the true rulers of the world. The Old Ones being evil because they show casual disregard towards lives of humans and other humanoids makes about as much sense as humans being evil because they show casual disregard for the lives of cattle and insects: none at all. Yes, I know that Faerun has many instances of Cartoony Good battling Cartoony Evil, but I don't care. We're not talking millennia ago as they suggest, we're talking countless and nameless aeons ago - and that means all familiar rules are out. I'd say this is Bat Country, but bats are also mammals and too familiar compared to lizards... So, welcome to Lizard Country.
Then, the whole Aribeth angle needs work. Instead of "hot paladin chick goes herpaderp blackguard because her lover got fragged", give it a bit of leeway for fuck's sake. See, it's basically about a misguided follower of the Maimed God realizing that Justice is an illusion after slamming into an example of Just Sacrifice face-first. Tyr was just, because he is a) lawful and bound by honor and tradition b) good i.e. willing to sacrifice (his hand etc.) for the greater good. Aribeth thought that she could and should follow that ideal, but was proven wrong. Facing that failure, she could develop by either:
a) Dropping the good part of her alignment and becoming a zealot; good i.e. the acceptance of personal sacrifice for others is a weakness, and only rigid adherence to strict immaterial axioms can provide strength. Mortals succeed or fail in the degree that they manage to know and follow the immutable laws of the world, which are not the world but which govern and create it (as Lyric Suite teaches us). Oh, and by the way, since the lizardmen have been in the world for so much longer and have created huge parts of it so long ago, they are much more in tune with those immutable laws, and the only truly Lawful path is to join the lizardmen cause...
b) Dropping the stuck-up lawfulness and dedication of her past life; becoming a good-natured, carefree strumpet, living day to day, seeking pleasure and having no greater joy than sharing pleasure with others. The more the merrier! After all, we're all going to die soon and there's absolutely nothing you can do about it, so we might as well spend our final days in joy and explore the unbounded thrills of the senses while we can...
Let's see, what else? Oh yeah, instead of "Where does Evul lie?", make the central theme along the lines of time, decadence and mindless entropy. D&D is all about exploring and looting forgotten ruins of an ancient golden age, whose downfall was the very thing that made it possible. Lizardmen were once at the apex of power, beings of boundless ability creating reality out of whims because they could - and now they're not only sub-divine but also sub-human (remember, the equivalent of sub-canine in human terms), mindless brutes dreaming of past glory their devolved brains can't even comprehend anymore by night, and rooting in the mud for sustenance by day, cowed by beasts they once hunted or created. They are Dead but Dreaming, for their dreams are brief glimpses of glorious past, and their waking is an undecipherable veil of oblivion... Humans might be near the mortal apex now, but they are also doomed to the same fate - as is any other intelligent race in Faerun. With the right dialogue choices, the party can make Elminister and Drizzit realize the tragicomic futility of existence and commit suicide (finally!).
Then, this whole lame "short-term power leads to evil, long-term wisdom leads to good" angle implied in the lizardmen amulets? You must be kidding me. No, let's go with: sticking to misguided human (manboon) ideas of "good" and "evil" will only lead to failure. In fact, there is no way to stop the lizardmen from destroying the world - because they are its true creators and rulers, and the world must take part in the destiny of its creators. Faerun can only be a chaotic swamp wasteland ruled by scaly idiot gods - and the fact it hasn't already become such is just a short-term (in cosmic terms) fluke and result of foreign influences that cannot hold (because they are foreign and unlawful, from the underlying cosmic perspective). The only way to win the game is by being amoral and gathering raw power, ascending above foolish human notions of morality. The words of power etc. can serve that function well enough, as long as they are not lizard-created: they are eternal, and the lizardmen are just one of the countless races that has learned them since the dawn of time - and they used them to create their Paradise, which briefly slipped away from their grasp, but will now return to its rightful rulers... The party can only survive by tapping into (being infected by) that primal source of power and creating their own world and paradise, in which they are as gods. Who cares about shitty Faerun and its lizardmen rulers, when you have your own private world in which your slightest whim becomes real thanks to the immense powers you wield? Victory is sweet, glorious, and ours...
The epilogue happens in the far future of that player-created world. The party members are of course still alive despite the countless and nameless aeons that have passed - because in this world they created, they are effectively immortal. However, time, decadence and entropy must always take their toll - and the party members are now beings of divine power, but oblivious to their nature - idiot gods too far gone to even remember they posses divine power, chewing lichen from trees and particularly slow bugs to survive, while their previous prey and creations prance about ruling the world, lulled by the illusion of sentience... And only rarely do they hear the mad piping of Azatoth, laughing at the colossal joke of existence...
P.S. And then one of the party members awakes on a slab in Mortuary, having forgotten the whole story...
P.P.S. This post sponsored by my tequila vendor.
P.P.P.S.