I've never played Drakkengard, though I wish I kind of had. That said, it is NOT required to play to get the gist of the Nier series. At best, you'll get very minor references through out both which you'd really only understand if you were hardcore into both series and were specifically trying to look for the little tidbits.
A singular ending to Drakkengard is what leads to the Nierverse, and that's pretty much all there is to it.
Now... I
have
played Nier, and am currently working through my third Nier: Automata playthrough (of which you need to play through, like, 5 times in order to really get the whole story), and between the two, I really like the original ever so slightly better. I love the characters just a bit more, their voice acting, and overall setting, and the music in the original is still my favorite OST for a video game ever. Nier: Automata is absolutely amazing though, and against most other contenders would still be my game of choice.
For the record... You DO NOT have to play Nier in order to play Nier: Automata either. They are related only in the sense that they are of the same world, but the gap in time is huge and there's nothing much that you take away from Nier in the hopes of it being answered or revisited in Nier: Automata.
Think of it this way.
Drakkengard = Final Fantasy 6
Nier = Final Fantasy 7
Nier: Automata = Final Fantasy 8
They are all similar in that they share a lot of things, like common names, chocobos, potions, etc... But that's pretty much where the similarities end. Drakkengard/Nier games are pretty much no different.
