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Invasion of Arcavios // Invocation of the Founders

Whenever you cast an instant or sorcery spell from your hand, you may copy that spell. You may choose new targets for the copy.

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, search your library, graveyard, and/or outside the game for an instant or sorcery card you own, reveal it, and put it into your hand. If you search your library this way, shuffle.

March of the Machine

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When Invasion of Arcavios enters the battlefield, search your library, graveyard, and/or outside the game for an instant or sorcery card you own, reveal it, and put it into your hand. If you search your library this way, shuffle.

March of the Machine Promos

Damage dealt to a battle causes that many defense counters to be removed from it.

A Siege’s controller can’t be its protector. If a Siege’s protector ever gains control of it, they choose a new player to be its protector. This is a state-based action.

A battle can be attacked by all players other than its protector. Notably, this means a Siege’s controller can attack it.

A battle can be dealt damage and be target of spells and/or abilities that target “any target.”

A battle’s “defense” is displayed in the bottom right corner of the card. A battle enters the battlefield with that number of defense counters. If another permanent enters the battlefield as a copy of a battle, it also enters with that number of defense counters.

A copy of a spell is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger.

As a Siege enters the battlefield, its controller chooses an opponent to be its protector.

Battles can’t attack or block, even if one also becomes a creature. If an attacking or blocking creature somehow becomes a battle in addition to being a creature, it is removed from combat.

If a Siege never had defense counters on it (perhaps because a permanent became a copy of one), it can’t have its last defense counter removed. It will be put into its owner’s graveyard. You won’t exile it or cast the other face.

If a battle has no defense counters, and it isn’t the source of a triggered ability that has triggered but not yet left the stack, that battle is put into its owner’s graveyard. This is a state-based action. This doesn’t cause a Siege’s intrinsic triggered ability to trigger.

If a battle that’s being attacked somehow stops being a battle, it is removed from combat. Similarly, if its controller changes in the middle of combat, it is removed from combat.

If a non-battle permanent that is already on the battlefield become a copy of a Siege, its controller chooses one of their opponents to be that battle’s protector. However, it will most likely be put into its owner’s graveyard because it has no defense counters (see below).

If a permanent that is represented by a transforming double-faced card becomes a copy of a Siege, it will be exiled as that Siege’s triggered ability resolves, then it will be cast transformed. Note that this applies only to transforming double-faced cards, not to modal double-faced cards that can normally be played using either face.

If a token or a card that isn’t represented by a transforming double-faced card becomes a copy of a Siege, it can’t be cast as its triggered ability resolves. It will remain in exile. If it’s a token, it will cease to exist the next time state-based actions are performed.

If the spell has damage divided as it was cast, the division can’t be changed (although the targets receiving that damage still can). The same is true of spells that distribute counters.

If the spell that’s copied has an X whose value was determined as it was cast, the copy will have the same value of X.

If the spell that’s copied is modal (that is, it includes a choice from a bulleted list of effects), the copy will have the same mode. A different mode can’t be chosen.

If you copy a spell, you control the copy. It will resolve before the original spell does.

In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game comes from your sideboard. You may look at your sideboard at any time.

In a multiplayer game, if the protector of a battle leaves the game and that battle is not currently being attacked, its controller chooses a new protector for it as a state-based action. If it is being attacked, its controller chooses a new protector for it once no creatures are attacking it. This means that it continues to be attacked and can be dealt combat damage as normal.

Only creatures controlled by a battle’s protector can block creatures that are attacking that battle. This means a Siege’s controller can never assign creatures to block for it.

Sieges each have an intrinsic triggered ability. That ability is “When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost.”

The copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).

You can’t choose to pay any alternative or additional costs for the copy. However, effects based on any alternative or additional costs that were paid for the original spell are copied as though those same costs were paid for the copy.

You don’t have to declare up front where you’re going to search. You may search your library, pause, sigh, check out your graveyard, frown meaningfully at your opponent, then finally grab a game-winning sorcery card from your sideboard. Bit dramatic though.