If all of the damage is prevented, Mjlnir's effect no longer applies
The fact that Nghathrod just gets it for you, regardless of how expensive it is, is beyond my wildest dreams
At instant speed you can pay to make X 1/1 Citizen dorks and your stuff gets indestructible for the turn, which is a nice variant on your answers to mass removal, and this is a strong combat trick where you can cast this after an attack your way, make a bunch of 1/1s, block, and keep them alive, or you can just use this in response to a big sweeper like a foe's Wrath of God when you only have two for the X to save your team and get in a Commander bump
So, if you want to have a bit of comedy in your act, take someone's powers away, or have an ongoing bit in your act, PRISON DECK is for you
Theyre essentially gates for a different subset of payoffs