Githyanki
I would probably go with a high level Githyanki NPC - they are very intelligent and have Psionics abilities in addition to their class levels. With the right/appropriate magic items or spells, once could enable transport to the shadow plane for your plot, and with the right psionics, probably control or compel another character or your NPC wizard. They may not even need a magic item to enable the shift:
2.0 Monstrous Manual: "All Githyanki have the ability to plane-shift at will. They will rarely travel back and forth from the Astral to the Prime Materal Plane."
But, it sounds like they have the option to go anywhere.
As far as why a high level Githyanki might be messing about in the shadowShadowFell plane - apparently their deity is a lich-queen and "no Githyanki has ever progressed beyond 11th level... Githyanki that reach 12th level of experience are immediately drawn out of the Astral plane and into the presence of the lich-queen where their life-force is drawn out to feed the ravenous hunger of their cruel demi-goddess"
So, I don't know if an 10-11th level Githyanki will fit the bill for you, but I think they are fairly underused. His business on the shadow planein ShadowFell could have something to do with commands from the lich-queen. In the background he might even be thinking about his own personal survival, circumventing his fate or even slaying her. Could be some interesting twists there.
Also, Githyanki hate, hate, hate mind flayers, having been enslaved by them before. I noted your assassin used mind-flayer based poison, and that made me think of them a bit.
Level appropriate magic items for such a high level Githyanki might include a ring of spell turning. An intelligent, deadly opponent that plane shift in from the Astral to the Material plane at any time could certainly put the willies into a high level mage and threaten him to do whatever he wanted, especially if he had such an item.