|I wasn't running a beta one, but it's appropriate.|
While building the deck, I realized that I just didn't have the cards to make it a proper suicide black. A lack of Juzam sealed that fate, though I considered substituting Juggernaut (Derelor was sadly out of the question). Stone Throwing Devils just weren't feeling it for me, and without Hymn, the Dark Rit's weren't nearly as good as turn 1 cards. So instead, I went the control route, and realized, I needed an actual win condition.
I looked at my black beaters (under valuing artifact creatures like a noob), I made a list, with Nightmare at the top, other contenders included Sengir Vampire, Fallen Angel, Demonic Hordes, Lord of the Pit, and finally Mold Demon. I evaluated each of their strengths and weaknesses, and concluded Nightmare would make the best of the list, which it became a two of (though I wish I found room for Sengir, maybe next online event).
Nightmare became a quick 'answer creature', finding himself typically settling down in a nice cottage (and maturing some great life for me), going to the disco, or facing God himself. When he didn't befell one of these awful fates, he typically hit hard and fast, and even the -1 from animate wasn't that big a feal.
Edit: Someone pointed out he also can't be Control Magic'd, which is a plus I didn't think of.
If I could go back, would I run him again? Maybe not. There were games where I had some wimpy Nightmares, and while I never encountered an Armageddon, that doesn't change the fact it's very much possible. Further more, there are better artifact creatures at 6 mana, though not as strong at the raw power as him.