Hi. :)
I have been running my new system with the Z390 Dark for a few weeks now.
9900K is delidded (direct die with der8auer OC-Frame) and running at 5,2@1,38 (-25 vdroop) but I felt temp could be better.
So I sanded the CPU die slightly with some very high grit paper (2500) and after a little while temps finally were reasonable (3 rounds of sanding and testing).
After the final round of sanding and testing I ran the CPU for a couple of hours gaming and running some benchmarks (3Dmark, Cinebench and so on) and the temps were much better.
Then I decided to check what max temps would be so I wanted to run Prime95.
As soon as i started running the small FFT test the system froze and upon reboot it will stop at post code 7F with no image on screen and then reboot after a few minutes.
 
When looking in the manual it just states that 7F is reserved for future use (not that helpful)

I have tried using safe boot and reset CMOS, also tried using one of the other BIOS, nothing helps :(
BCLK was left at 100 and XMP was 3200MHz.
I have also tried leaving system unplugged from the wall for a few minutes.
Starting to think that sudden CPU load may have killed the CPU, borrowing an i3 from a mate tomorrow to test.

Either your cpu is dead, or your os has corrupt files.

I ran into an issue where the direct die was not putting enough pressure on the socket. Try swapping back to the standard clamp socket and put your IHS back on. Hopefully that is your issue.

We'll, putting back the stock socket is easy and definitely worth a shot. I was having an issue where my system would not post anything over 3200mhz on my RAM, no matter what I did. Luckily, someone posted on Reddit once they installed the direct die their system would no longer post at the same RAM profile as before. So I switched back and low and behold, I was able to post at my 3600 XMP.

dallemon
Sajin: Not the OS as system doesn't even finish post. :)

Oh, so then how did you try safe boot?

Hope your problem get solve

See Post Below for corrected version https://forums.evga.com/FindPost/2926456

dallemon
So I remounted the original socket clamp, still gives 7F post code with my CPU.
If I try with the borrowed CPU (8350K) I get an image, but machine doesn't finish POST, stops with some weird flickering between post codes, but the most visible is 04 which seems to related to CPU microcode.
So pretty sure my CPU is dead, but will have to test in a different mainboard just to be sure.

Let's hope your CPU is OK.

EVGATech_LeeM

bcavnaugh

dallemon

Sajin

dallemon
Sajin: Not the OS as system doesn't even finish post. :)

Oh, so then how did you try safe boot?

It's not Windows Safe mode, but the motherboard has a button called safe boot, it's for booting with default settings if an OC is unstable. :)

It is a Small Black Button on Some EVGA Motherboards BIOS Safeboot Button" (Corrected)

 
Some also have this on the Back I/O Panel as well but it is a Clear CMOS Button and is Red. (Corrected)
Another reason why I like EVGA Motherboards, but some of the Lower End Motherboards do not have this Little Red Button.
Like the Old Jumper that you would Short Out to reset the Bios back to the Defaults.

 
Someone is not researching before Posting
The same as the Clear CMOS Button & Clear CMOS Jumper
[....]

Actually, you're confusing the Safeboot button with the Clear CMOS Button.  The Z390 DARK has one of each (technically, two Clear CMOS buttons). 
 
First EVGA board to have one.  The Safeboot button is black.
 

Thank you for the update and correction, I was in a hurry due to another posters lack of knowledge of EVGA Motherboards
Corrected Image, kind of hard to see the Color of the Button in the image.

dallemon
So after further testing, my CPU is dead.
But I got my board to boot with the borrowed CPU, I did another remount of the clamp socket and after this it booted fine with the 8350K, no weird flickering post code :)
Going to buy a new 9900K on the 28th and try again :)
I'm so happy I didn't have to RMA my board that I have waited so long for. :)

But Bummer about the CPU

you think it possible particulate matter got to the underside of the cpu and caused a short?  From all that sanding?  Has to be some fine dust around somewhere.