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Commander
When Lord Xander, the Collector enters the battlefield, target opponent discards half the cards in their hand, rounded down. Whenever Lord Xander attacks, defending player mills half their library, rounded down. When Lord Xander dies, target opponent sacrifices half the nonland permanents they control, rounded down.
Card advantage
Magecraft — Whenever you cast or copy an instant or sorcery spell, each opponent loses 2 life and you gain 2 life. +1: You lose 1 life. Look at the top three cards of your library. Put one of them into your hand and the rest into your graveyard. −3: Each opponent sacrifices a creature with the greatest power among creatures that player controls. −8: Each opponent may discard a card. If they don't, they lose 3 life. Repeat this process six more times.
(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.) I — Each opponent sacrifices a creature or planeswalker. II — Each opponent discards a card. III — Put target creature or planeswalker card from a graveyard onto the battlefield under your control.
Card advantage already have
Suspend 2—{1}{B} (Rather than cast this card from your hand, pay {1}{B} and exile it with two time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost.) Search your library for a card, put that card into your hand, then shuffle.
Control / removal
Protection from permanents with corruption counters on them +1: Each opponent loses 2 life and you gain 2 life. Put a corruption counter on up to one other target creature or planeswalker. −3: Gain control of target creature or planeswalker until end of turn. Untap it and put a corruption counter on it. It gains haste until end of turn. −7: Gain control of each permanent with a corruption counter on it.
Control / removal already have
Finisher
Necrogoyf's power is equal to the number of creature cards in all graveyards. At the beginning of each player's upkeep, that player discards a card. Madness {1}{B}{B} (If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)
Flicker Xander
Soulbond (You may pair this creature with another unpaired creature when either enters the battlefield. They remain paired for as long as you control both of them.) As long as Deadeye Navigator is paired with another creature, each of those creatures has "{1}{U}: Exile this creature, then return it to the battlefield under your control."
Indestructible As long as your devotion to blue is less than five, Thassa isn't a creature. At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control. {3}{U}: Tap another target creature.
Land
Land already have
Mana ramp already have
Protect and Hit
Sacrifice & triger
Casualty X. The copy isn't legendary and has starting loyalty X. (As you cast this spell, you may sacrifice a creature with power X. When you do, copy this spell. The copy becomes a token.) +1: Each opponent loses 2 life unless they discard a card. If you control a Demon or Devil, you gain 2 life. −2: Create a 1/1 red Devil creature token with "When this creature dies, it deals 1 damage to any target." −7: Target player draws seven cards and loses 7 life.
Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.) At the beginning of your upkeep, return target creature card from your graveyard to the battlefield. At the beginning of each opponent's upkeep, that player sacrifices a creature.
Sorcery
Choose one. If you control a commander as you cast this spell, you may choose both. • Target opponent chooses a creature they control. You gain control of it. • Choose a creature you control. Each other creature you control becomes a copy of that creature until end of turn.
Triger discard
Sideboard
Equipped creature has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token that's a copy of that creature that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.) Equip {4}
{R}, {T}, Discard a card: Create a token that's a copy of another target creature you control. It gains haste and "When this creature dies, draw a card." Sacrifice it at the beginning of the next end step. Activate only as a sorcery. Blitz {1}{R} (If you cast this spell for its blitz cost, it gains haste and "When this creature dies, draw a card." Sacrifice it at the beginning of the next end step.)
As Molten Echoes enters the battlefield, choose a creature type. Whenever a nontoken creature of the chosen type enters the battlefield under your control, create a token that's a copy of that creature. That token gains haste. Exile it at the beginning of the next end step.
Myojin of Grim Betrayal enters the battlefield with an indestructible counter on it if you cast it from your hand. Remove an indestructible counter from Myojin of Grim Betrayal: Put onto the battlefield under your control all creature cards in all graveyards that were put there from anywhere this turn.
Changeling Whenever you cast an instant or sorcery spell, if it targets one or more other permanents you control, create a token that's a copy of one of those permanents. When a spell or ability an opponent controls causes you to discard this card, create a token that's a copy of target permanent.
You may have Undercover Operative enter the battlefield as a copy of any creature on the battlefield, except it enters with a shield counter on it if you control that creature. (If it would be dealt damage or destroyed, remove a shield counter from it instead.)
When Woe Strider enters the battlefield, create a 0/1 white Goat creature token. Sacrifice another creature: Scry 1. Escape—{3}{B}{B}, Exile four other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.) Woe Strider escapes with two +1/+1 counters on it.
Maybeboard
+3: Destroy target noncreature permanent. −2: Gain control of target creature. −9: Nicol Bolas, Planeswalker deals 7 damage to target player or planeswalker. That player or that planeswalker's controller discards seven cards, then sacrifices seven permanents.