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Fortified Position holds the battlefield, and one Flurry before the attack takes it

Issue #350 (a slice of #200), 2026-10-14. Constructed, Duel (485). Rules version 2026-07-16. Card text verbatim from data/corpus_flat.txt; rules pasted from data/Riftbound-Core-Rules-2026-07-16.txt. Read the unopposed clock for the baseline this is measured against.

Subject: fortified-position-mutated-mouser-shield-lasts-the-combat, the Calm ENGINE in which OGN-279 Fortified Position grants [Shield 2] for a combat, and UNL-036 Mutated Mouser, a 1-Might body with printed [Shield 2] and [Tank], defends as a 5 when the grant goes on it. Walked as a game, the wall held. On their T3 two attackers with 5 Might between them hit two Mousers. The grant was the difference between keeping the battlefield and an empty one: with it they lost both attackers and you kept a Mouser and the battlefield. The entry's own answer clause gets the timing wrong. OGN-133 Flurry of Blades cannot kill a Mouser "in response to the very trigger this engine exists to fire". By then the Mouser is a defender and a 3. It kills them in the opponent's Main Phase, before the attack, when every Mouser is a 1. Cast there, one Energy takes both Mousers and the battlefield.

Why this line. scripts/sequence-pick.mjs --unplayed, re-run on 2026-10-14 at 3ff0f0b, reads 773 entries, 262 with forced-ordering language, 102 in the queue and 37 of them with no play, unchanged from the issue. The pick is by global rank, skipping entries that already have a play: this entry is rank 66 (a hit on before), second in the list. The other entry of this slice is rank 65, SFD-007 Gem Jammer and OGS-011 Flash moving units both ways, played in Flash keeps the bodies, and both battlefields go with them.

1. Needs

Legendany legend with Calm among its two domains (103.1.b.1); here OGN-261 Radiant Dawn (Calm/Order)
The battlefieldOGN-279 Fortified Position (colourless), as one of your three
The bodiesUNL-036 Mutated Mouser (Calm, E2, M1)

Neither card is Signature (data/cards.json), and neither is in data/legality.src.json.

The card text the play turns on:

2. When a Mouser is a 5, a 3 and a 1

Shield counts only while the Mouser is a defender. 814.1.c: "While I am a defender, I have +X [M]." 814.1.d: "Being a defender means the Unit has gained the Defender designation during Combat." Out of combat a Mouser is a 1. Every Mouser is a 3 as a defender, the printed Shield 2 on its Might 1.

The battlefield's grant is a Defend Trigger, so it comes after the designation. 383.4.f.2: "These Triggered Abilities are put on the Chain as Pending Items after the Unit these effects correspond to gains the Defender designation during Combat." By the time the grant is on the chain the Mousers are already 3s. When it resolves, 814.2 sums the two: "If a Unit has Shield, or has been granted Shield, and is granted Shield by an additional source, the Shield Value of all granted Shield keywords is summed." The chosen Mouser is Shield 4, a 5. The grant lasts the combat, and 814.1.d.1 keeps it as long as the designation: "Shield remains in effect as long as the Unit maintains the Defender designation."

Only the controller gets the grant. 190.6.d: "“You” in a battlefield’s abilities refers to the battlefield’s Controller". You have to control Fortified Position when they attack it.

[Tank] decides where their damage goes. 815.1.b: "I must be assigned lethal damage before any other unit with the same controller as me that does not have [Tank] during the Combat Damage step." With two Mousers, 815.1.c.2: "If more than one unit with Tank is present with the same controller in Combat, damage may be assigned to any of them."

3. Their order: Body/Chaos, bodies and an attack trigger

SFD-203 Battle Mistress (Body/Chaos): "When you recycle a rune, you may exhaust me to play a Gold gear token exhausted. When one or more enemy units die, ready me." Nothing they play costs Power, so they recycle no rune, and readying the legend does nothing in the walk. Its Chosen Champion carries the Sivir tag. They play UNL-097 Kinkou Initiate (Body, E3, M3), UNL-092 Demacian Diplomat (Body, E2, M2) and OGN-130 Crackshot Corsair (Body, E3, M3): "When I attack, deal 1 to an enemy unit here." That trigger chooses, so it targets (355.10).

4. The turns, going first

Calm/Order against Body/Chaos. Your battlefield, Fortified Position, is FP; theirs is B. Your opening four are two Mutated Mousers and two cards for later turns. SFD-038 Ribbon Dancer is Calm, E3, M3. Every card either side plays in the walk costs Energy only, so neither rune deck's split matters to it.

turn    runes   your turn                                                 points
T1      2       Mutated Mouser (E2) to base.                              0
their T1        Kinkou Initiate (E3) to base. Three runes, none left.
T2      4       The Mouser walks to FP, Conquer: +1. A second Mouser      1
                (E2) to base. Two runes left.
their T2        Kinkou Initiate walks to B, Conquer. Crackshot Corsair
                (E3) and Demacian Diplomat (E2, 1 XP) to base. Five
                runes, none left.
T3      6       Hold FP: +1. The second Mouser walks to FP. Ribbon        2
                Dancer (E3) to base. Three runes left.
their T3        Hold B. Corsair and Diplomat walk from base to FP: 3 + 2
                against two Mousers. Corsair's trigger: 1 on the first
                Mouser. FP's grant: the second Mouser is a 5. Their 5
                kill one Mouser; your 8 kill both attackers. FP stays
                yours. Seven runes, all ready.
T4      8       Hold FP: +1.                                              3

Their points: 0 after their T1, 1 after their T2, 2 after their T3. After your T4 it is 3 to 2.

Why they did not attack FP on their T2. Kinkou Initiate's 3 against one Mouser: the Mouser is a 3 on its printed Shield and a 5 with the grant. Their 3 on a 5 are not lethal, and its 5 kill Kinkou. B was empty and worth a Conquer.

Their T3, the combat. Both walk in from base in one move (144.3), and the cleanup after the move stages the combat (323.9). When it opens, 464.2.c.3 gives the two Mousers the Defender designation, and each is a 3. Two triggers are pending: Corsair's attack trigger and Fortified Position's defend trigger. 464.2.e.1: "The Attacking player, who has Focus, places Triggered Abilities on the Chain first, followed by all non-Defender players in Turn Order, followed by the Defending Player." The Corsair's 1 is aimed at the first Mouser before you place the grant, and 340.1 resolves the newest item first: "The newest Finalized Chain Item resolves." The grant goes on the second Mouser and resolves first. Then the Corsair's 1 lands on the first Mouser.

The damage step, 465.2.c: "Starting with the Attacker, each player assigns an amount of damage equal to their summed Might among the other's Units." They have 3 + 2 = 5. The first Mouser is a 3 with 1 marked and needs 2 more. The second is a 5. Both have [Tank], so 815.1.c.2 lets them pick. Either they put 2 on the first and 3 on the second, and the first dies, or they put 5 on the second, which dies, and the first lives with 1 marked. One Mouser dies either way. You have 3 + 5 = 8, and 3 on the Corsair plus 2 on the Diplomat kill both. Your surviving Mouser keeps Fortified Position, and the Combat Cleanup heals it (466.1.a.1). Had you put the grant on the first Mouser instead, it would be a 5 with 1 marked, needing 4. Their 5 kill either Mouser but never both.

What the grant bought. Without it both Mousers are 3s, one with 1 marked: their 5 are 2 + 3 and kill both. Your 6 still kill both attackers. Nobody is left, and 466.5.b: "If there are no Units remaining here controlled by any player, the Battlefield becomes Uncontrolled." You lose the Hold on T4: 2 to 2 instead of 3 to 2, and you have no Mouser left. The grant's +2 is exactly that margin.

How often this happens. Fortified Position has to be the battlefield the game uses. 485.5: "Each player randomly selects one (1) of their three (3) Battlefields." Tournament Rules 402.1 makes the three unique names, so that is 1 in 3. The draw pile is 39 cards, the 40 of Tournament Rules 601.1.b less the Chosen Champion, which 103.2.a.1 places in the Champion Zone. Going first you have seen 5 cards by T1 and 6 by T2 (four opening, one draw a turn). At three Mousers, one by T1 and two by T2 is 5.64%. Times the battlefield, the walk's line is 1.88% of games. One Mouser by T1 alone is 34.52%.

5. Breaks to

The cheapest break is OGN-133 Flurry of Blades (Body, E1), cast in their Main Phase before the attack. "Deal 1 to all units at battlefields." Out of combat every Mouser is a 1 (814.1.c), so 1 is lethal (142.4.b). 370.1.a.2 kills both at once: "An event can occur simultaneously with other events only when those events are all the result of the same Game Action or change in state occurring." Fortified Position is empty, and the next cleanup in the Open State takes it (323.6). Walked on their T3: Hold B, 2. Flurry: both Mousers die, and Kinkou Initiate at B takes 1 and lives. Corsair and the Diplomat walk into Fortified Position unopposed: Conquer, 3. You have no Hold on T4, and it is 2 to 3 after their T3, for one Energy. With three copies in a 39-card pile, it is in their hand by their T2 (6 cards seen) in 40.30% and by their T3 (7 seen) in 45.73%. On their T2 it kills the one Mouser at FP the same way, and Kinkou Initiate walks into FP instead of B.

Inside the combat the Flurry kills nothing by itself. The moment the grant is on the chain (383.4.f.2), every Mouser is a defender and a 3, so 1 damage does not kill one. Cast in response to the grant, the Flurry puts 1 on each Mouser and 1 on each attacker. In the walk's combat that makes the first Mouser need 1 more, the second 4 if the grant is on it, and the Corsair and Diplomat need 2 and 1. Their 5 then kill both Mousers (1 + 4), and your 8 still kill both attackers. Nobody is left, and Fortified Position is uncontrolled (466.5.b). Cast in the combat, the Flurry still costs you both Mousers and the battlefield, but both attackers die with them. Cast before it, they keep both attackers and conquer.

The scope of that verdict. A Body/Chaos deck plays only Body, Chaos and Body/Chaos cards (103.1.b.1). The cards in those domains in data/corpus_flat.txt whose text reaches all units, all enemy units, each unit, all other units or all friendly units are ten. Flurry of Blades is the cheapest that removes two Mousers from Fortified Position:

What answers the Flurry. OGN-045 Defy (Calm, E1 + 1 Power): "Counter a spell that costs no more than :rb_energy_4: and no more than :rb_rune_rainbow:." Flurry of Blades costs E1, and Stare Down E2, so Defy counters either. On their T3 you have three ready runes from your T3. SFD-045 Not So Fast counters only a spell that chooses a friendly unit, and the Flurry chooses nothing (355.10.d).

The entry's Order protection covers their T3, not their T2. With Order as the second domain, the entry lists OGN-243 Darius, Executioner and OGS-013 Garen, Commander: "Other friendly units have +1 :rb_might: here." A Mouser beside either is a 2 out of combat and lives through the Flurry. Each costs E6 + 1 Power. That is six runes, one of them exhausted for the Energy and recycled for the Power (164.2.a, 164.2.b), and you have six on your T3. A unit need not be played to base. 355.2.a: "By default, Valid locations include the controller’s Base or a Battlefield the controller controls." Cast on your T3 straight to Fortified Position, either one stands beside the Mousers before their T3. Entering exhausted (178.1.a.1) does not switch off a static ability, and the Flurry walked above kills nothing. It costs your whole T3: the second Mouser still walks in, but Ribbon Dancer is not played and no Defy is held. It does not cover their T2, when the first Mouser stands alone at Fortified Position, and the Flurry is in their hand by then in 40.30%.

6. Verdict

Right about the grant and its duration, wrong about when the cheap answer lands. On defence, the grant made one Mouser a 5 for the whole combat. Their two attackers died and you kept the battlefield, where without the grant nobody would have been left. The entry says the one-Energy sweep can land in response to the grant. At that point every Mouser is already a 3 and the sweep does not kill. The window is the opponent's Main Phase before the attack, when a Mouser is a 1, and there one Flurry of Blades takes both bodies and the battlefield. In Calm, the answer is OGN-045 Defy, held up on their turn.

7. Not verified

No card in the walk costs Power, so it rests on no rune-dealing assumption for either side, at any split. Defy needs one Calm among the six runes you have on your T3. With six Calm in the twelve the six hold none in 1 of 924 deals, 0.11%. Darius or Garen, cast on your T3, needs one Order among the same six runes. With six Order in the twelve that fails in 1 of 924 deals, 0.11%.

I put Flurry of Blades in their deck at three copies and did not walk Defy in the turn table. I did not walk Fortified Position as their battlefield instead of yours, a third Mouser, the T4 attack on B, the mulligan, or 2v2. The probabilities in §4 count only cards in hand at the turn named.

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