Mikaeus, the Unhallowed works just as well as Melira, Sylvok Outcast and doesn't require running green, just costs more mana. Just a trade off of another color in the deck for higher mana cost, but if your sac engine is Ashnod's Altar then you have the mana off the first sac.

Sigil Captain gives the best damage output, since Murderous Redcap would enter as a 3/3 every time off his ability but that requires 2 extra colors, more viable with Phyrexian Altar as your sac engine.