The definition of a nightfiend?

Alright, I know they're mandrake squad leaders, but what's the definition? The 40k wiki definition is an ancient and powerful mandrake, but I don't realy trust that its from a wiki.

Are they like a sort of alpha male, a lord maybe?

I would say a really strong mandrake.

As for really what it is, we really dont know. GW has really left mandrakes very open ended, and as for a society or civilization structure, we dont know. Heck, we dont even know what they made a deal with (I mean is it a daemon??? or some enity from the warp/webway/whatever it is) But that doesnt stop you from making stuff up

It is an evil creature that likes the night

So any dark eldar?

Powerful / purebred mandrakes I guess because they refer to 'packs' of mandrakes which implies a more archaic species and they wear a patchwork of flayed skin as their only clothing so it comes across to me as roughly a species of supernatural savages for want of better phrasing so I very much doubt a civilisation, but they get supplied the weapons by Commoragh organisations as I don't realy see them having any sort of manufacturing ability.

As for where they come from, I have a theory of the haemonculi covens performing an arcane ritual or something to make them into the beings they are a bit like scourges. They obviously started out as dark eldar as they still need to feed on pain and they also have weapons reminisent of surgical tools.

That's my theory.