Prison decks do a great job of making your opponents feel completely helpless while not quite closing the door. Many players derive a great deal of joy from watching their opponents try to find a way out of a hopeless situation, but never manage to get there.
This deck revolves around a single spell, and it better be a powerful one since it specifically states that you won’t be able to cast any more spells. Enduring Ideal isn’t cheap to cast, but in Modern where you have access to so many powerful enchantments, the power level is through the roof. If you can survive to 7 mana and cast this thing, you begin to establish a lock that your opponent won’t be able to break out of.
The heavy hitters come from some extremely expensive enchantments. Form of the Dragon is the main kill condition. This will set your life total to 5, so you’re at risk against burn and flying creatures, but ground creatures can’t attack you. You also get to deal 5 damage each turn, and your life total will reset, so shooting down flyers is a way to keep yourself safe. Dovescape removes all reasonable ways to interact with your plan. If your opponent tries to resolve a noncreature spell, it will dissolve into Bird tokens. Killing your enchantments or trying to burn you out is no longer an option, and many combo decks cease to function. The 2 cards together present a soft lock, but you need to watch out for the Birds attacking you.
This deck has tons of ways to stop creatures from pressuring you, and early on in the game you will rely on Ghostly Prison to minimize the number of creatures that can enter the red zone. So many decks rely on going wide with small creatures, some operate on one land, and Ghostly Prison can shut them down. Sphere of Safety is Ghostly Prison #5 which is often more powerful after an Enduring Ideal, and Ensnaring Bridge can make sure the large creatures (and eventually all creatures) stay away from your life total.
You have tons of early creature interaction. It starts with Oppressive Rays tying up their mana and creatures. Journey to Nowhere can get rid of a problematic creature, while Oblivion Ring will handle tough permanents. Runed Halo can do lots of work in the early stages, and Suppression Field will put a damper on powerful abilities in Modern (including fetchlands).
Porphyry Nodes keeps the battlefield under control. If your opponent has a 1-drop on the play and you lead on Nodes, they can either keep playing creatures that are going to be destroyed, or let you buy a ton of time. If your opponents need to take a turn off from playing a creature while also trading 1-for-1 at a minimum, you’re coming out of the exchange extremely far ahead. If you happen to have your 1 Gideon, Ally of Zendikar, you can make tokens each turn when your opponent doesn’t play anything, and choose not to when they do to help control the battlefield.
Leyline of Sanctity is insane against all of the black discard and red burn decks in the format when you can play it to start the game. Later on, it’s still effective to make sure you can’t be combo’d or burned out after resolving your Ideal. Phyrexian Unlife is yet another way to make sure you can’t be killed, and creates a near lock with Form of the Dragon.
With an effectively mono-white main deck, you’re looking to get a ton of enchantments into play early and often. These will slow down your opponent and allow Nykthos, Shrine to Nyx to power out an Enduring Ideal to win the game!