Thank you! good to be here
hmm, good points... however let me try to explain my thoughts here:
i gave him an invulnurable save to keep him more in line with the original (5++), and same goes for the I6 (in fact, the only difference in his statline is the sorely needed toughness boost). reason being that he needs some incentive to actually charge into combat and not just sit around bale-flaming people, having no saves of any kind in close combat would be quite the deterrent
the "Hunter of heads" rule is a nightmare to word in a sensible way honestly, but the idea of this specific version is that the opponent is not allowed to back down from the challange, the way i see it Khar' would just go straight for the yet-to-be-flayed skull of his choice rather than challange in the conventional sense
I was actually originally planning on giving him rending for simplicity, but then i would still have to add some sort of instant death effect on rends for flavor and to keep things in line with the original, besides, i did not want him to become a tank hunter with S6 and rending, so instead i opted for this, which is a fairly simple way of getting the desired effect with very little confusion.
2-5 units of mandrakes seems better though, yes, you are right, its not like they are an expensive tax anyway if you just want the "spiderman from hell".
as far as overpowering go, that is totally not what i went for, and i know that mistake very well, +1 strength and a soulblaze worth giving a toss is not as huge as it might seem, in addition, for the utility of the nightfiend you do have to buy the 10pts upgrade for him, he has to be alive, and you need to actually inflict a wound on the target unit for soulblaze to take effect.
The idea here was to actually give the mandrakes a role that would mesh with the exsisting dark eldar forces in that classic synergistic eldar way, providing something else than dark lances or a hailstorm of infantry killing, with the ability to harass and disable instead (infantry through special rules, and transports by sniping at rear armor), overall sporting a support role