(Theory) Nocturne is the original master of Shadows and was freed by Zed

When Zed opened the box a weakened Nocturne who wasn't back go his full power yet was freed and since he was only a spirit at this point and still rusty from years of imprisonment he had no choice but to take over Zed's young and fit body.Nocturne perhaps was someone who got corrupted by the darkness until he became what he is today.He was then imprisoned by Shen's father and the other twilight warriors.When he took over Zed's body (causing him to scream as his spirit got corrupted) with he used his power over shadows and killed Shen's father as revenge for all those years of imprisonment.But to do this he had to unlock the darkness inside Zed.

Shen couldn't help his father because of the shroud of "darknesssss" masking his father's distress.When Nocturne was finished he knew the warriors of the Kinkou clan would eventually overpower him and cleanse him from Zed reimprisoning him in his torturous chamber.Nocturne fled to look for a source to regain his power quickly,which was dwindling without a host.Before leaving Zed he saw true hatred in him and he let him live with the power/teachings of the darkness.Zed was to carry on the legacy of the forbidden art as a sole successor.Nocturne later found the Field Architect as a host with enough clearance to get him close enough to the Nexus and was soon able to burst back into minimal power which could only be limited by a physical form.To gain his true power back he had to find people with darkness inside of them and kill them to harness it.To gain the most dark power from them as possible he terrorized them.

Another fact to support this is that Nocturne was probably imprisoned for a long time and he saw how much the world had changed which caused him to say "this place disgusts me" one of his quotes.Nocturne was perhaps imprisoned by the original twilight warriors as he says "their twilight approaches" to mock them and to hint at his power growing for every victim.

Additionally,Akali could be a potential shadow apprentice. "Akali,the fist of shadow"