Pretty self explanatory. Be Khemri (the one with Settra) and have 5 Tomb Guardian units in the same army. Different from other races is the way you can recruit units. With the Tomb Kings they don't cost you anything, but you can only recruit as many units as you have buildings build of their troop chain. So you will be required to have enough territory to be able to recruit them. But that should not be a problem by the midgame.