I Used to play warhammer fantasy with an army of Tomb Kings.
That got discontinued right before Age of Sigmar was released.
I switched over to 40k after that dark day and have wanted to convert at least Settra over to the necrons for awhile.

The basic back story I have for him now is,
Some necrons came to the old world, and region that Settra resides through the warp.

Since the Tomb Kings magic is inherent in them, and the Necron scout troop didn't have any real defences aginst magic, it was just a matter of time before the necrons where over run.

Due to the magic of this old world, the Necrons were unable to phase back to a crypt world and consequently, kept reassembling after each encounter in Settra's region.


...long story short, Settra eventually strikes a bargain with the necro leader of the scouting unit promising a cease fire and shelter till the necron can return in exchange for an alliance with them if and when they return to the necrons timeline

In order for Setra to command any necrons he had to graft the scout leaders arm to his body.
The command protocols would now be able to transmit through Settra via his new arm and Necron scepter.