:warning tl;dr and nerd alert:
back to the lore, I am going back to the start of the warhammer world.
A race of space faring demi-god-like creatures known only as the Old Ones finds the current world a frozen ball of ice. generally believed it was ruled at this point by really big creatures, giants being on the small side for the time. the ancestors of the dwarves, elves, humans and lizards all exist but none can even read and write at this point. The Old Ones make their home Lustria (south America) one of the few places at the time that is actually warm and they begin changing the world. they move its orbit, move mountains create seas and so forth. they also begin civilizing the aforementioned races. The lizardmen become their servants, elves learn magic, dwarfs how to work steel and the humans learn the fine art of fart jokes (they aren't really civilized too much). over the next couple millenia the lizardmen wage war in their millions destroying some species and fighting others back to where they are now. The old masters such as dragons, dragon shaggoths and Sky Titans find their realms severely reduced for various reasons, if they aren't exterminated outright.
now the Old Ones used magic and so could their servants and various other individuals, but it was very controlled and generally considered safe. to facilate easy travel and access to magic the Old Ones create Warp Gates at either pole. Think a magical Stargate. the lore isn't clear on what happens next or why. but as things start settling down, the Old Ones hurriedly create some new races, Ogres and Halflings, who are supposed to be resistant to mutation because that is the draw back of humans. Suddenly the warp gates are destroyed and Chaos is introduced to the world. it is unclear if chaos was always an enemy of the Old Ones or what is happening off of this planet but suddenly the four chaos gods are here. just as suddenly the Old Ones are gone, not sure if they are killed by the blast, they gtfo, or if they are mutated or sucked into the warp or what. they are just gone and Chaos and chaos rule. with the warp gate destroyed Warpstone is produced (it is generally assumed the warp gates were made of this but some suggest warpstone came from the warp and caused the explosion), basically warpstone is pure refined chaos in a glowing green stone. Warpstone chunks fall all over the globe and can still be found today all over, all the warpstone was created in this instance. not only did some fall back to earth but enough was kicked up so that a second, evil, moon was formed that lingers to this day and it will occasionally rain warpstone down on the world.
when the warp gates collapse infinite numbers of daemons, raw chaos/magic personified in the forms of one of the various gods of chaos, Khorne the berserker, Nurgle the disease spreader, Tzeentch the Great Changer and Slaanesh the defiler. at this time Humans have a very few scattered civilizations but nothing like today. the elves and dwarves while strong don't really have a clue about what is going on. and the chaos daemons rush the world, but specially Lustria gets the worst of it. The Lizardmen were the only real power at the time but with their direct connection to the Old Ones they are a little lost, and many of their leaders, called Slann (most powerful mages in the body of a toad) are killed by the blast so even the great Lizardmen empire is shaken. Huge battles take place all over the world, dwarves are pushed into the moutains, elves to their island the humans are scattered. The Lizardmen and Daemons fight climatic battles all over the world, battles in the millions some lasting for decades. even though the Lizards had the most power and could almost literally repopulate in endless numbers the Daemons are litterally endless and with some much magic/chaos running rampant all over the world the Daemons are unstoppable in their numbers and using smaller warp portals they can show up almost anywhere. the fighting goes on long that the non-lizardmen races forget the old ones even existed. to give a sense of scale it is implied that the whole continet that Naggarond sits on (North America) is fought for litterally inch by inch with the daemons constantly pushing the south. (an aside I forget, before the explosion of the warp gates the southern pole was connected by land to the rest of the world, but afterwards the water levels rise cutting it off. here be monsters and daemons)Several times the Lizardmen fight to the gates but can find no way to close them; and while the Lizardmen can spawn almost infinitely their leaders the Slann can not, they are a fixed number so every loss of one is huge blow in this war of attrition.
So the daemons have pretty much one, everyone is pushed back into their last strong holds when the elves start a magical ritual to create a drain for all the raw magic floating around sustaining the daemons. Through their magical senses the Slann figure out what they are doing and aid them from another continent with the elves not knowing it, basically problems with the drain fix themselves in the eyes of the elves). The daemons start freaking out and go ape**** on everything but just as defeat looks certain the drain is complete and 99.999999% of the extra raw magic in the world is drained away. (there is always some level that is natural, what was drained was the extra spilling out through the warp gates). making the drain sinks a good chunk of Ulthuan (the middle portion) and kills most involved, including the most powerful Slann.
peace reigns for a bit while the various races crawl out from whatever rock they were hiding over. but the world is changed forever with the introduction of chaos. and some species emerge and grow strong before they can be checked in this time, mostly the various breeds of Orks, goblins and their monster pets while beastmen are introduced from the warping effects of warpstone and the chaos magic that saturated the world for so long.
the lizardmen are devastated by the war. They completely changed their home rain forests into death traps to fight the daemons, living rivers to eat armies and bunches of crazy stuff. most of the Slann go into a long slumber after this for a couple reasons. the constant fighting for centuries not being the least. during this time they commune and try to figure out the Old Ones plans for the world, thinking if they can right the world and get rid of chaos the Old Ones can or will return.
with very little Slann leadership most of the Lizardmen take some steps back from what they were. and while Lustria was the main home the Lizardmen have several other places in the world that survived, the Southlands (africa) and some at various other points on the globe. but with so few Slann left, and most sleeping/coma/communing the Lizardmen lose even more from degradation as most places have no Slann. At one point in the Lore the Southlands still have Slann but in the last two Lizardmen rule books there aren't any, so those Lizardmen that survive become savages simply living in the ruins with no connection to what they were.
The Slann eventually figure out they can't remotely fix the problem so at the time of the game they are starting to mobilize their armies for direct intervention seeing how some species are in direct conflict with the will of the Old Ones, Chaos, Beastmen and Skaven. The Lizards are down to 5 cities I think so they can't muster the full armies that they used to could (sorry the good ole boy in me got out). Some of the Slann are still wicked strong (one of them thinks the Edge of the World Mountains are in the wrong place so he moves them around a bit creating the disaster for the dwarves) and the oldest one still alive gets the oldest Lizard alive to start whooping some but.
The Lizardmen aren't generally at war with any of the "good guys" but they all keep bugging them and stealing their stuff, think the conquistadors to the Inca and Maya, so conflict is common for the men who have settled in Lustria. they are however actively trying to kick out the bad guys and recover some of their lost lands and relics from the old ones from all over the world.
the lizardmen go through some other major downfalls, specifically because of the skaven so there is some major hate there as well. for the men of the empire it is not uncommon for them to travel over find what they think is an abandoned temple loot it, return home and find the Lizardmen after them. who often/usually don't want or take most of it back but generally just want a few very important relics.
The Lizards should play a lot like chaos warriors. very good heavy foot soldiers with some very interesting monsters and various allies.