Which system is the Neverwinger Nights Diamond Edition based on?

Thanks!

NWN1: 3.0
NWN2: 3.5

nwn 1
is
3.5 or beyond depending on which group of dedicated scripters you're running with.

Like PeterScott said, NWN1 is based on a modified version of 3.0 and NWN2 is a modified version of 3.5. Some things are different in the games than the real D&D versions so you might rather want to check the game's wiki pages rather than D&D pages.

For NWN1: http://nwn.wikia.com

For NWN2: http://nwn2.wikia.com

I think Beirut is trying to say that nwn1 can also be played using other rules (like 3.5 or 4 or 5 or others) through agressive scripting. But the actual core mechanics are, as said above, 3.0 with modifications that make it easier in a quasi real-time environment.

There was a mod:
https://www.gog.com/forum/neverwinter_nights_series/dandd3_5_for_nwn1

+Pool of Radiance: Ruins of Myth Drannor = 3.0
+Icewind Dale 2 = 3.0
+Temple of Elemental Evil = 3.5
+Knights of the Chalice = d20