"The Slann form one of the core species of the Lizardmen race, created by the Old Ones to be the leaders, organizers, architects, and techno-mages of their society and the backbone of their culture. Being one of the most magically powerful races on the face of the Warhammer World, these favoured servants of the Old Ones have considerable intellect and magical abilities beyond the minds of most mortal races, and rule the Lizardmen as a venerated caste of semi-religious magic-users. Not physically menacing - their bodies are toad-like with large heads and bulbous eyes - with a flick of their fingers, Slann can topple cities, engulf foes in flames or open vast vents in the ground below. Enemy wizards find the mightiest incantation they try to employ unraveling before them as a Slann contemptuously waves it's hand.
There were five spawning of Slann by the Old Ones, each with a particular role to play in their Great Plan. No new Slann have spawned since the departure of the Old Ones. Without any new Mage-Priests to pass the torch of their generation, they are a dying race, faced with extinction. Over half of their kind died in the Great Catastrophe, including all of the First Spawning - the wisest and most powerful of the Slann, and the only ones that had direct contact with the Old Ones. In the ages since, many other Slann have died violently — irreplaceable losses that are greatly lamented. With each Mage-Priest lost, the Saurus and Skinks further insulate those that remain, protecting them with their lives."
- From the Warhammer Wiki