Um. One mana for a one-sided Honden of Night's Reach? That was the + ability on the five-mana Liliana Vess. The three-mana version Liliana of the Veil had to make each player discard, and even then it was Standard-warpingly strong. (See the changes R&D were making everywhere around Khans of Tarkir to cope with LotV being in M15, before they gave up and cut her because she was warping too many things with powerful discard.)

The middle ability looks very strong too... Perhaps you meant to type 3BB and got distracted partway through?

Have you learned nothing from Tibalt? For a 1 mana walker to be balanced, it has to suck. No one likes cards that suck.

­Kytheon, Hero of Akros is, in some ways, a 1-mana walker. He doesn't suck like Tibalt.
I do feel that Ob is extremely unlikely to see a 1-mana incarnation.

Kytheon has a large setup cost to balance it. Plus, unless you're dropping two Ornithopters to transform him on turn 2, you're still paying more than 1 mana.

What direction would you take Ob Nixilis if the story idea was for him to be a pawn of Ulamog, destroying Zendikar soul by soul?

I don't think that plot makes sense for Ob.

Why? He's intent on destroying Zendikar, which makes him blind to other things, the easiest way for him to destroy Zendikar is to ally with Ulamog.

He's mono-black. He would never work for someone more powerful than himself. He might use the services or ally with someone weaker, but only because he knows he can kill them as soon as they stop being useful. I suppose in his origin story, he used the Chain Veil to make a deal with the onnake demon, but 1 his only other choice was to die, and 2 this was before he was a planeswalking demon.

Liliana is mono-B and has worked for Bolas and Kothophed, although she proceeded to kill Kothophed. Maybe Ob Nixilis' inflated hunger for power and revenge has clouded his senses into believing he can kill Ulamog after Zendikar is dealt with?

Liliana is most definitely regretting that decision now. It also doesn't make sense to use the same plot for different characters; that's sloppy.

I changed the card, but today, Void Winnower was spoiled, and its flavor text suggests Ob has gone over to the Eldrazi.

No? Ob has always enjoyed wiping out life, so it seems to me that he's just learning new methods of genocide by watching the Eldrazi.

I suppose that's a fair interpretation as well.