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What is the order of operations when a Tyrant Guard uses its Shieldwall rule to intercept damage taken by a Hive Tyrant with the warlord trait "Adaptive Biology"? Assume that the Hive Tyrant had been wounded in a previous phase or turn, so that Adaptive Biology is active when the Hive Tyrant gets wounded. Additionally, what happens when either the Hive Tyrant or the Tyrant Guard have the psychic power "Catalyst" active on them?


Relevent rules from the Tyranids codex (none of which appear in the current Tyranids FAQ):
"SHIELDWALL: Roll a dice each time a friendly <HIVE FLEET> HIVE TYRANT loses a wound whilst they are within 3" of this unit; on a 2+ a model from this unit can intercept that hit – the Hive Tyrant does not lose a wound but this unit suffers a mortal wound."
"ADAPTIVE BIOLOGY: From the end of the first phase in which this Warlord suffers any wounds, for the remainder of the battle when inflicting damage upon the Warlord, reduce the damage of the attack by 1 to a minimum of 1."
"CATALYST: Catalyst has a warp charge value of 6. If manifested, select a friendly TYRANIDS unit within 18" of the psyker. Until the start of your next Psychic phase, each time that unit loses a wound, roll a D6; on a 5+, the damage is ignored and the unit does not lose that wound."


Example 1:
Hive Tyrant with Adaptive Biology (previously triggered) receives 2 hits that successfully wound, causing 3 damage each. A unit of 3 Tyrant Guard are within 3" of the Hive Tyrant. The Hive Tyrant fails its saves vs both hits.

Option A: Adaptive Biology reduces both 3-damage hits to 2-damage hits. The Hive Tyrant should take 4 damage, losing 4 wounds. But the Tyrant Guard activate their ShieldWall rule, and all 4 rolls of 2+ are successful. The Hive Tyrant loses no wounds, and the Tyrant Guard unit receives 4 mortal wounds. One Tyrant Guard (with 3 wounds) dies, the other takes 1 wound.
Option B: Since the Shieldwall rule transfers "hits", a roll is made for each 3-damage hit. Both 2+ Shieldwall rolls are successful. The Hive Tyrant takes no wounds, and the Tyrant Guard unit receives 6 mortal wounds, so 2 Tyrant Guard die.
Option C: As option B, but each Tyrant Guard receives only one mortal wound from each hit. The Hive Tyrant takes no wounds, and the Tyrant Guard unit receives 2 mortal wounds, killing none of them but leaving one with a single wound remaining.


My opinion is that the rules support option A. Rationale: The Tyrant only "loses a wound" after damage is applied. The damage would be reduced by Adaptive Biology before being applied.
My opponent was of the opinion that option B was correct, since the damage was never actually applied to the Hive Tyrant (being negated by ShieldWall before application), and further that Shieldwall mentions the Tyrant Guard "intercept HITS" (my emphasis). I argued that if so, it should really be a toss-up between options A and option C; and since option C is clearly unfair (3 damage being reduced to one mortal wound), then the correct option must be option A. Unfortunately, my opponent insisted on option B, and I lost the dice-off (and 2 Tyrant Guard) :(


Searching YMDC turned up only a couple of threads about Shieldwall, none of which addressed Adaptive Biology. One thread suggested that the "simultaneous rules" rule allowed the player whose turn it is to determine the order of special rules. When the Hive Tyrant is being wounded during the Shooting Phase, that would be the shooting player. This could imply that the opponent could insist that Shieldwall activate before Adaptive Biology.


Example 2:
As above, but the Hive Tyrant has activated Catalyst successfully on himself.
(I would assume that if Adaptive Biology works before ShieldWall rolls, then so would Catalyst).

Example 3:
As Example 1, but the Hive Tyrant has successfully cast Catalyst on the Tyrant Guard.
(I would assume that Catalyst would work on any mortal wounds taken by the Tyrant Guard as a result of successful ShieldWall rolls).

Sequencing. Whoevers turn it is picks the order the rules are resolved. So yes, the shooting player could resolve the Shieldwall activating before Adaptive Biology.
I misread. Adaptive Biology will reduce the damage before the damage is inflicted, while Shieldwall happens AFTER the damage is inflicted.

Adaptive Biology would always kick in first. You can’t suffer a wound (trigger for Shieldwall) until you’ve resolved damage, and you can’t resolve damage until you know how many points of damage are taken, which Adaptive Biology modifies.


Automatically Appended Next Post:

 BaconCatBug wrote:
Sequencing. Whoevers turn it is picks the order the rules are resolved.

Maybe you should read the triggers for the rules more closely. Not all are simultaneous.

As noted Adaptive Biology comes first. Both Catalyst and Sheldwall trigger off the same thing, the Tyrant losing a wound. The Sequencing rule some into effect, with the player who's turn it is deciding which order you activate the abilities.

Not optimal since the that means you'll likely resolve it one one on the Tyranids's turn and the other way their opponent's turn, but that is the rules.

Thanks for the replies everyone!

Hadn't considered the sequencing effect on Catalyst and Shieldwall. Will have to cast Catalyst on the Tyrant Guard instead of on the Tyrant himself: then the sequencing should not be a problem.