Sotek is the Serpent God of the Lizardmen in Warhammer Fantasy Battles. He's also known as the god of the Skinks, who worship him in particularly large numbers; a small number are born with a special red crest that marks them as favoured servants of Sotek.
The weird thing about Sotek is that he isn't one of the Old Ones, an alien race that basically created the Warhammer World as we know it. However, after the Great Cataclysm (in which the forces of Chaos broke into the world and tried to mess it up), all of the Old Ones disappeared/left and were soon enough worshipped as sort-of gods by the Lizardmen.
Sotek is different, oddly so; a god born (like those of the other races) from the collective thoughts, beliefs, and emotions of the Lizardmen and, in particular, the Skinks. The Slann can't decide if Sotek is part of the Great Plan of the Old Ones, which the Slann are trying (and mostly failing) to carry out. This has the Slann very worried indeed.
However, Sotek is a fairly recent discovery. You see, the Skaven of Clan Pestilens decided to invade Lustria, the home of the Lizardmen. They fouled up a couple spawning pits, decimated a temple-city, and generally made a mess of things, all in the name of the Horned Rat, their god of plague, warfare, and... well, rats. A crazy Skink named Tehenhauin led a mass exodus from the destroyed temple-city, with a very special (and unique) plaque in hand, prophesying that a new god would force the Skaven from Lustria. And, lo and behold, the plaques were right. Tehenhauin and his host (and a shitload of snakes that wandered from the jungle) fought back against the Skaven and pushed them back to the ocean, at which point Sotek (or something the Skinks took for Sotek...) chased the Skaven into the sea.
Since then, Sotek has been the primary patron god of the Lizardmen race and, in exchange for a few Skaven hearts here and there, grants the Lizardmen nifty powers and helps them beat up more Skaven. After driving the Skaven out of Lustria, he (allegedly) went after them in their tunnels, which is why no more mass invasions have occurred. In the End Times he did nothing except for one thing; Sotek beat up the Horned Rat so badly, the Horned Rat didn't join the Chaos Gods when they finally destroyed the world.
Interestingly, the twin-tailed comet is seen by the Lizardmen as the sign of Sotek, representing his tongue. It was present at the birth of the God and is a great omen to the Lizardmen. The Empire of Men see the comet as the symbol of their god Sigmar (as one was also present on the day of his birth) and the presence of it always heralds the birth of a new powerful hero (see: Valten). What does this mean?
Well, probably nothing. Oh well.
It is also interesting to note that, prior to 8th edition, Sotek actually was one of the absent Old Ones, although he was seen as the most active one and the fluff about Tehenhauin was roughly the same (with the notable exception of Sotek's actual manifestation to drive the Skaven into the sea).
With the coming of Age of Sigmar there have been theories about Sotek being Dracothion, the Celestial Drake, and the one who helped Sigmar restart the universe, maybe, maybe not. This may actually make a lot more of sense (and awesome) to the rather sketchy story about Sigmar getting saved by a magical space dragon out of nowhere, and for that matter, Dracothion helping the Lizardmen to settle in the new reality in high Azyr, a place where Chaos can't reach. Not to mention how the twin-tailed comet is both the symbol of Sigmar and Sotek. Very suspicious indeed. The book Hallowed Knights: Black Pyramid has human worshippers of Sotek in it, though elaborates little on his place in the Mortal Realmss.
Sotek sounds very similar to/is an alternative spelling of Sobek, the crocodile-headed god of rivers from Egyptian Mythology, who appeared as part of the Mulhorandi (fantasy Egyptians) pantheon in the Forgotten Realms setting of Dungeons & Dragons. Much about Sotek is probably a blend of the Aztecs' gods of Quetzalcoatl (a serpent deity) and Huitzilopochtli (an asskicker hummingbird-associated god that required his followers to perform sacrifices for him to give him the energy for more asskicking).