If you do, put two +1/+1 counters on Hei Bai
Grimgrin acts as both a removal engine and a growing threat, combining sacrifice synergy with commander damage
Sacrifice effects, edicts, and mass removal help Zombies survive longer games where boards get crowded
Granted, it's at sorcery speed instead of being able to do it on someone else's turn to start off a Storm combo with a bit of a boost, but in all likelihood it's still going to do more for you in a mono-black build than a simple Dark RitualDark Ritual will, for the same mana cost
Even at twice the cost, Time Spiral is every bit as powerful as Timetwister in the right circumstances
Lets focus on Yorion, Sky Nomad for a moment