I think the creature generation effect should be 100% due to the high cost and the fact that it only happens when it kills a creature, but it should be limited to only work with non-Death and non-Darkness creatures (basically, it doesn't create creatures when it kills Death or Darkness creatures, or maybe it can't target these creatures at all), keeping it aesthetically (and somewhat mechanically, keeping it from being OP with a boneyard out) balanced.