[Suzerain of Sheol]

I'm leery of collapsible swords, having owned a few, they really lack the structural integrity of forged blade, mostly good for surprise. Not to mention if I understand the design right, he'd have to construct it at the beginning of a fight, no? Unless he feels like lugging it around put together. I'm assuming those cutouts on the back edge are to reduce weight and improve handling? Should make it at least usable without being a superhuman.

The second one is a lot more elegant to me, though why not give him both? I don't think you'd even need an explanation for the small one, it should be pretty simple to come by a weapon like that. As for the big one... maybe he got it from artificer friend/contact at some point? They're the ones who'd make something like that. Or it could have belonged to a demon maybe, some kind of hell-steel?