Ultimate SpellBuffCost: 6
Sacrifice all units you control.
Your non-Demon heroes level to max and become Demons. Put two +1/+1 runes on each, they get readiness, and are invisible until they leave play.
BlackDemonology Ultimate Magic

Card-Specific Rulings
Heroes generally don't have types, so your Demonology hero is NOT a Demon until Metamorphosis makes her a Demon. Once she is a Demon, playing a second Metamorphosis will not affect her. If she leaves play, she stops being a Demon. — Sirlin
Whenever a hero leaves play, it always loses all properties such as levels, damage on it, +1/+1 runes on it, etc. So it also loses all buffs it got from Metamorphosis. — Sirlin
Invisible Rulings
"Sneaking past" means you can ignore patrollers when attacking and attack anything you want. You can also attack the patrollers if you prefer. — Sirlin
When something invisible attacks an enemy unit or hero, that enemy unit or hero DOES deal its combat damage back as usual. — Sirlin
If an opponent has the Tower add-on and their tower has already used its once-per-turn detect on something, then you attack with something invisible, the Tower will not deal its damage to your invisible thing. It would if it's once-per-turn detect hadn't been used, but it can only damage things it can see. — Sirlin
You can [target] your own invisible things whether you have a detector or not. — Sirlin
Readiness Rulings
Readiness doesn't let you ignore any other attacking rules. You still can't attack with something that's exhausted, and you still can't attack with it the turn it comes under your control (unless it also has haste.) — Sirlin
If you attack with something that has readiness, it won't exhaust. You CAN exhaust it to pay for the cost of a spell or ability (or as an effect from a spell or ability), even though you can't attack with it again that turn. — Sirlin
Things with readiness can only attack once per turn, even if an effect would normally let them attack again. When cards leave play and then come back (for example, from Geiger or Pasternaak's max level abilities), they count as new objects though. So it IS possible to attack with something with readiness, have it leave play from one of those abilities, return, and if it has haste it could attack again. — Sirlin