Could the old man be the sorcerer Chrome the ninja mentioned. Maybe He accidently summoned Saitou like all the items that were strewn about the labyrinth.
I checked chapter 25.1~25.3 again, the assassin looks too different so they're probably different people. they're probably co-workers or something and the demons decided to join up?
I checked the raws assassin is the same person in the earlier chapters also we don't know how long its been this chapter and his introduction story so that is why his hair style is different.
Shit. I didn’t realize this was the Assassin from some chapters ago that fell in love with the Witch and befriended demons... I really liked him. I don’t know what to think anymore or how I want this to end.