Void is a solid card, but doesn't fit the theme. The only thing that's got me truly interested in this deck is the theme, haha. I have no idea why I dislike Grixis colors/generals so much but I do.
Phyrexian Etchings is an experiment that I haven't had much of a chance to try out. In theory it looks like a not quite as broken
Necropotence - amazing in a deck that can afford to let it get to 4-5 age counters, which this deck can definitely do. I'll be sure to report back once I get some reliable testing in.
It's also a lot less threatening than
Necropotence, so that's a point in its favor too. I think the key to using it effectively will be knowing when to stop paying for it.