A Starstele is a type of Necron 'structure' that house the formidable Transdimensional Abductor. The reason why 'structure' is put in quotations is because they, like most Necron constructs, has translocation protocols, which allows them to teleport away or around the battlefield...like a vehicle...as such, Starsteles are in this very vague spot like the other Necron constructs with these translocation protocols such as the Sentry Pylon and the Necron Pylon.
When three of these structures are grouped together, they form what is called a Convergence of Dominion, which is a Necron formation akin to a Imperial Guard's Armoured Fist Squad. There are currently three models of these structures, each with a level of degradation and decay that show either the passage of time or the particular Necron Dynasty's lack of proper maintenance.
Crunchwise, the Starstele and the Convergence of Dominion are one and the same as the structure cannot function to its fullest extent by itself. The constructs are first and foremost, a unit support buffer that gives surrounding Necrons significant leadership bonus. As such, it is recommended to spread these structures out to maximize their cover distance and sphere of influence. The rules even state it out flat, in which each model within the construct's influence is treated as a separate unit. They will give 2 leadership to any unit within 6" and in most cases, it should increase your Necrons to leadership of 12. So essentially, they make your most basic Necron not short-circuit when something bad happens, sort of like a giant Wi-Fi router, with guns. As aforementioned, these structures can move, albeit with help from a Cryptek. But their movement range is infinite, so are you going to honestly complain?
Their Transdimensional Abductor is a nasty weapon. The weapon is an Assault D3 weapon with S4, AP-3, D3 stats. They should be able to easily clean up any horde infantries. However, the only downside is their pitiful range of 12". Combined with their general immobility (Outside of the translocation protocols of course) and the need to distance themselves from each other, these structures can be surprisingly quite vulnerable if left unattended.