What is a Nightmare Grind?
A Nightmare grind is actually a combination of a hollow grind, flat grind and upper swedge grind. 
The hollow grind is on the belly of the knife and the tip is a flat grind for strength. The hollow grind typically meets with an upper
swedge grind and at this point there is a triangle the complete thickness of the blade. This triangle has been nicknamed the “Triangle of Doom”.

Mick Strider once said that a certain knife was a nightmare to grind and the term stuck.

Examples of the Nightmare Grind:

Gen IV SnG, Gen II SnG, Gen I SnG, MSC Nightmare Tanto SnG (left to right)
(Photo by HKSIG45)

MSC Nightmare Droppoint SMF (Damascus)
(Photo by HKSIG45)

"Triangle of Doom"
MSC SMF Recurve w/ Trisula and Wiremesh Finish, MSC SnG Nightmare Tanto, MSC SMF Recurve (left to right)
(Photo by Patrick_B on USN)