"Another fine victory over the crude and the disorderly, Obyron! It seems strange to me that the enemy generals saw fit to loose so many wild beasts upon the field of battle. Their so-called tactics become more desperate and barbaric by the day. So long as we stand tall, old friend, it shall avail them naught. Onward!"
- – Nemesor Zahndrekh, following his victory over a Tyranid splinter fleet
"Even if we all ceased to be flesh and blood millions of years ago, which of course I don’t believe for a moment,’ – Zahndrekh actually winked – ‘wouldn’t it have suited us better to live in denial of that, as some fools might say I had done? Wouldn’t it be better, Obyron, just to accept our fate, and enjoy immortality for the everlasting life of merry campaigning it has proved to be?"
- – Nemesor Zahndrekh to his Vargard Obyron revealing the fact he isn't as insane as he seems
Nemesor Zahndrekh (AKA "that old coot" or "Creedcron") is generally accepted to be the most bro-tier Necron overlord in the galaxy. He's a mix of Creed and Old Man Henderson, with a healthy dose of Don Quixote to go along with it. Among a thousand other commanders who'd rather exterminatus an entire sector just to see their foes dead, old man Zandrekh makes a point of employing only honourable methods of war, giving his foes a chance to surrender, and capturing them alive if possible.
Zahndrekh's prisoners can look forward to a pleasant stay on his flagship, having weekly dinners with their captor and witnessing the marvels of Necron culture and technology... until the old coot's bodyguard gets rid of them claiming "they were killed while trying to escape" (considering the denizens of this galaxy, they probably did do something to bring it on themselves). Speaking of said bodyguard, he's the only other Necron who can stand Zahndrekh's company, and has the doubtlessly cool-sounding title of Royal Vargard (regardless that Vargard is the word "Vanguard" with a few letters changed).
Why He's Such A Cool Guy
Well, Zahndrekh did not take well to the biotransference process. In fact, it gave him Alzheimer's. As such, he is completely unaware that he's an immortal robot from outer space, and thinks he's still a Necrontyr, from before the Eldar even existed. He's also nearsighted, making him see all of his enemies as Necrontyr rebelling against the Triarchs, which in turn makes him quite a bit less xenocidal than most of his species. Woe the day he finds a pair of glasses, and discovers the green, smelly and burly necrons he keeps meeting around the galaxy are not what they seem.
It's been revealed in the newest story containing him and Obyron that it's more Robo-Dementia and he is a bit eccentric. His awareness of the world around him comes and goes and he's somewhat aware of his condition and tries to make the most of it (eg; taking advantage of his condition to fool his enemies into underestimating him), which is both awesome and tragic, making for a very compelling character. A small expansion to his character has also been made through the form of a small cameo in The Twice Dead King series, where he is shown as someone known for his quirky (read: trollish) character as far back as the time of the flesh.
As for the "Don Quixote IN SPESS" aspect, it's actually canon – in 8th edition (Carried forward in to ninth, yay!), his Transient Madness rule has an effect named "Solarmills? Charge!" - just with infinitely less Fail, since his tactical acumen was untouched by his delusions. With a generous disregard for reality, swinging between pretense and belief that he's still in the past and a loyal servant who is aware of what's going on yet does nothing to stop him (compare Obyron with Sancho Panza), he's one hard-ass grandpa.
If you really want to see Zahndrekh in action, check out the Black Library Novella Severed. He and Obyron, a renegade crownworld, a hated ally, and some fun cyberpuppies.
8th edition is all about auras. Commanders and leaders projecting bubbles around themselves to buff up their troops so they can actually get their points back. Zahndrekh shuts them all down. Well, ok, only one of them down at a time, and only within 12 inches. But this is still huge. For instance, rules as written, you can shut down Azrael's Lion Helm, leaving the heavy plasma spam he was guarding open for some maximum rape.
On top of that, he gives cool random buffs (taken from a D3 chart) to one Infantry unit within 6", and you roll the D3 before you choose the unit to buff, meaning the buff won't be absolutely wasted just because of a bad dice roll (it can still be absolutely wasted, just not because of one bad roll). His random effects are +1 Attack, +1 Ballistic Skill, or "I'm Don Quixote! Charge those Solar Mills!" (i.e. re-roll failed charges).
Zahndrekh only comes with a Phase Shifter giving 4+ invulnerable Saves for extra survivability, or so it would seem. Alternatively, you can have Obyron perform a classic "GET DOWN, MR. PRESIDENT!" move to exchange a wound Zahndrekh would have taken for a mortal wound on Obyron on a 2+ roll if the two are within 3" (which sounds useless on its own since it sounds like Obyron would have to babysit Zahndrekh to get any use from it, but Obyron can also teleport himself and an Infantry unit within 6" of himself right to Zahndrekh in the movement phase). Certainly something to be used incredibly sparingly, especially since Obyron has a sizable point cost himself, but it could be useful to save Zahndrekh from what would have been a death blow.
Overall, though, Zahndrekh is most certainly far more situational compared to his big boss Sautekh HQ counterpart, Imotekh. The two only have a 15 point cost difference, and while the ability to shut down auras is powerful, Imotekh provides a lot more overall benefits to his troops, including a free command point if he's Warlord, being able to use My Will Be Done Twice, extra Living Metal regen, unique improved weapons (one being a direct upgrade to Zahndrekh's standard Staff of Light), and his badass Lightning Storm that has obscene damage potential (if it works, of course). But in the end, Zahndrekh's random buffs, synergy with Obyron, and aura shutdown can certainly provide a fun alternative playstyle.