The keys in your hand, the glasses on your head, the wallet in your pocket.
They were always there, waiting for you to remember them.
The Helvault, a dark, ineffable prison for Demons and Planeswalkers alike, made from a shard of the moon.
It was always there. It was always going to end like this.
Innistrad's luminous moon, it turns out, is eldritch in more ways than one. It's been up there taunting us ever since we knew the set name, shining its eerie light on countless evil machinations and Werewolf transformations.
Now it will forever hold Emrakul's enormous, ghastly form, shielding more worlds than one from her tentacle-inducing madness. We're left to worry and wonder for years whether it's really all that safe until the day it's inevitably not. Emrakul is gone, but I, for one, will never forget the host of Eldrazi horrors she created. "Latticework" used to be such a benign word.
Imprisoned in the Moon answers one of the current block's most pressing questions: Can Innistrad be saved from Emrakul? And how? With heroics from some familiar faces, that's how. Tamiyo's exhaustive research has paid spectacular dividends, as she's able to help the Gatewatch crack the case of what to do with an interplanar eldritch behemoth making nightmare monstrosities of humanity's former protectors.
But Emrakul's imprisonment leaves many more questions in its blue-tinged wake.
Can Emrakul not die the way Ulamog and Kozilek did, or is Jace finally heeding Ugin's warning that killings titans is a terrible idea? Can Innistrad ever recover? Does this mean Graf Rats and Midnight Scavengers are breaking up the band? Who is going to take care of Innistrad now that Avacyn is gone? Did they trap Emrakul in time to keep Liliana from turning literally everything on Innistrad into a Zombie?
Only time will tell.
We don't need time, however, to tell that Imprisoned in the Moon is a sweet Magic card.
The moon isn't terribly picky about the types of permanents it's imprisoning. It's happy to take care of lands, creatures, and planeswalkers alike. While Nahiri wasn't thrilled about the thousand years she spent imprisoned in the Helvault, she at least helped both Sorin and Wizards prove that Innistrad's spooky moon is capable of handling any pesky spark-ignited mages out there.
In every format, planeswalkers are threats that need to be answered before they're able to take over the game. Removal for them is scarce, particularly in blue's slice of the color pie. With help from Innistrad's moon, however, the days when blue had to take care of a planeswalker before it hit the board are over. Imprisoning them in the moon renders even the most powerful 'walkers capable of nothing more than producing a colorless mana.
Of course, planeswalkers aren't the only threat that the moon can effectively hide away. Creatures can't resist its ominous influence, turning over their stats and swapping their card type for the opportunity to be a colorless land. Blue mages can continue their rejoicing, because, unlike something like Sleep Paralysis or Claustrophobia, Imprisoned in the Moon prevents the creature's controller from using any of its abilities or casting a protection spell that might make the enchantment fall off. The moon and the Gatewatch's spells are so potent that they don't care about something as trifling as indestructibility.
With emerge, well, emerging as a new mechanic in Eldritch Moon, the fact that the creature is turned into a land also robs its controller of the opportunity to sacrifice it to summon something better. It's a best-of-both-worlds situation, as the creature is also still on the battlefield—albeit under eldritch disguise—so it can't be retrieved from the graveyard (a constant fear with the number of necromancers running around Innistrad) or fetched from exile.
With lands like Geier Reach Sanitarium and the most recent creature lands providing dominant late-game value, the ability to shut off threatening non-mana abilities on lands is only more upside on already quality removal.
Over the last few months, blue has slowly leached out of Standard's most effective control decks, with black and white giving better access to effective creature and planeswalker removal than the once-favored control color. Perhaps with a little help from the moon, blue can begin to reassert its claim to the control throne.
Going for a moonlit walk won't ever be the same, but I suppose Innistrad never was the safest place for Humans like you and me to wander at night, anyway.