All-in ramping into Curse of Misfortunes or Sphere of Safety on turn 2-4 is usually game-winning. Sphere instantly beats creatures decks but does nothing against other decks. Either way, it hopefully stalls long enough for us to draw Misfortune to actually win. Kruphix's Insight helps us draw these.
Mulliganing
We don't mulligan very well because we need 5 mana to deploy our finishers, so hands with fewer cards often don't have enough. Don't be afraid to keep hands with 2+ lands, 2+ ramp and one relevant enchantment (even if it's not a finisher, because we have tons of those); these are about as good as hands with 2 lands, no ramp, and threats.
Hands with Porphyry Nodes/Ghostly Prison, 2+ lands, and no ramp are still generally keepable against creature decks or game 1 because they buy us tons of time.
The London mulligan means we can tuck important curses if we've mulliganed at least once!
Gameplay
When to play Kruphix's Insight is a difficult decision. If we have relevant enchantments, ramp into/play them before Insight. If we have no relevant enchantments, 3 or less mana, and expect to untap 2+ more times, still play ramp before Insight since that lets us play a 5-mana finisher if we draw it. Even if we drew one off Insight, we couldn't play it next turn without ramping anyways. If we have nothing besides Insight, don't be afraid to turn 2 Simian Spirit Guide the Insight, since it'll often draw us ramp to replace the SSG.
Porphyry Nodes is very slow and will do nothing late game. Almost always play the Nodes as soon as possible (even if there's only one creature). Even if they take a turn off to let the Nodes die, that's at least a Time Walk and a removal. We'll draw plenty more threats later.
Fetch Mistveil Plains early since there's little downside, too late to fetch the same turn to shuffle back curses, and reduces our chance of drawing tapped lands. Don't do this against decks with land destruction if we need it.
Mainboard Discussion
Curse of Misfortunes is a one-card combo. If played turn 2-4, as early as turn 3-5, we get to fetch Overwhelming Splendor followed by Curse of Death's Hold to hard lock creature decks and activated abilities. Alternatives are Curse of Exhaustion to stop some combos or Curse of Echoes to stop spells (esp. counterspells). Once locked, we can fetch Cruel Reality and Curse of Thirst to finish the game as early as the turn Thirst comes down. Remember we get all of this from playing just one card, so we're still playing other removal, locks, and finishers in the meantime (unlike Enduring Ideal).
Sphere of Safety is usually a hard lock, with half our deck being enchantments, singlehandedly stopping many decks. It serves as copies 5-8 of Misfortune because it gives us unlimited time to win.
Utopia Sprawl, Simian Spirit Guide, Gemstone Caverns, Trace of Abundance, Overgrowth, and Pentad Prism are the glue that holds together the whole deck. They provide ramp, enchantment count for Sphere, and extra targets for our cantrips. Our 5-drops would be too slow if we weren't playing them turn 2-4 instead. They also make our mana so consistent that we're compelled to splash colors. We're actively good against Blood Moon (always fetch basic Forest), and can play through some land destruction (though obviously not too much).
Mistveil Plains lets us shuffle back any curses we accidentally discarded to Kruphix's Insight and Commune with the Gods. Mistveil is fetchable by 6 fetchlands. Mistveil also has the minor upside of ensuring we win the decking war if we somehow can't win with curses (e.g. Surgical, curses getting exiled, Leyline, Witchbane Orb, etc.).
Kruphix's Insight and Commune with the Gods are great because we're overloaded on enchantments and ramp, and not that much to do with it. It also digs into more ramp and our win cons.
Leyline of Sanctity auto-wins against Burn, Pyromancer (left with only 10 spells!), Titan Shift, Storm (game 1), and is still our best way to interact with noncreature/creature-light decks like Death's Shadow, other discard, Jeskai Control, Lantern, Ad Nauseam, etc.
Nether Spirit gives us a chump-blocker every turn since we have no other creatures in the deck. This works particularly well with a prison effect.
Oath of Jace (fetchable with
Kruphix and
Commune) is the best way to discard curses we've drawn, so we can shuffle them back with
Mistveil. Since have 9 all-color mana sources, it's fairly trivial to splash a color that we'll rarely need.
Liliana is an alternative discard outlet, but we've replaced her in favor of
Oath.
Sideboard Discussion
Greater Auramancy is important because, as rare as it is, enchantment removal is great at breaking our lock. Auramancy also synergizes with Starfield of Nyx to protect animated enchantments.
Sigil of the Empty Throne provides a backup top-end threat against discard/counter-heavy decks and extra finishers in rare scenarios where we have to race or can't win with curses.
Bitterblossom/Nyx-Fleece Ram/Lingering Souls are so good with any prison effect. Our opponent can often still send 1 or 2 through Ghostly Prison and maybe even Sphere of Safety. Just one blocker is enough to stop that indefinitely.
Specific comments Show
Bitterblossom is the most flexible of the bunch since it's a win-con, particularly good against control, and blocks indefinitely. However it's particularly bad with
Nodes.
The sheep is better against aggro since it soaks up some early damage and gains us valuable life against direct damage that can bypass our prison. However it doesn't block flying.
Souls produces the bodies immediately, giving a faster clock, and Kruphix's Insight draws half of one for free. However it doesn't count as an enchantment for Sphere.
Curse of Exhaustion is an auto-include since it's a tutorable Rule of Law
Curse of Echoes is also tutorable and is the only useful curse against control (it stops most counterspells, especially Cryptic Command bouncing our permanents) and some other heavily non-creature decks. Maybe Dovescape is just better though?
Nevermore synergizes nicely with Leyline to shut down the alternate win-con that most combo decks have (e.g. Primeval Titan after we stop Valakut, Empty the Warrens in Storm, etc.). It's also a one-card combo against some decks (e.g. Ad Nauseam, Death's Shadow, Gifts in Storm, Ironworks, Amulet Titan, etc.). Against removal-heavy decks, it's also great with Auramancy because we name their 4-of removal spell with Nevermore. Then they have find and destroy Auramancy with a different removal and still can never cast their 4-of.
Gemstone Caverns should be sided out if we're on the play, of course.
Starfield serves a similar function to Auramancy by reanimating destroyed/countered enchantments, allows us to attach curses through hexproof (e.g.
Leyline,
Witchbane Orb), and gives us a backup in case we weren't able to protect pieces with Auramancy. Starfield also synergizes with
Kruphix and
Commune to reanimate the enchantments we dump.
Ideal is not competitive in Modern; it's a 7-mana combo doesn't even win the game on the spot. The fact that
Curse of Misfortunes requires 2 fewer resources (lands/ramp) to cast and is always 1-4 turns faster makes Ideal obsolete. Let's look at it another way: if we survived 7 turns in Modern, doesn't that mean we're already winning? And if we're ramping with
Lotus Bloom or
Pentad Prism, why haven't we already won the game with Ad Nauseam?
Link
Check out my latest deck Claw Curse **Primer** , or Puca Curse, a more meme version of this deck featured by SaffronOlive.