Purge is a ranged attack. By default, ranged attacks require a target, and Purge says nothing to the contrary. So Purge requires a target. Only 1 target too, just like any other ranged attack. Just because it hits and damages many models does not mean it targets them all.
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Structures are valid targets for attacks. "Buildings" are not. So if before the start of the game you had agreed that the terrain piece was to be considered a Structure (IE destroyable), and not just a regular terrain piece, then it could be a legal Purge target.
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Erratum: Prime p.125 : Purge: Replace text of Purge with the following: The Bile Thrall sprays out the entire contents of its guts, deflating and automatically hitting all models within 6” of the Bile Thrall’s front arc. Terrain that obstructs LOS blocks the Purge attack. A model in range of the Purge attack cannot be hit by the attack if the attacker’s LOS to it is completely blocked by terrain. Models hit take a POW 12 damage roll and suffer Corrosion, then remove the Bile Thrall from play. Purge is a ranged attack.
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Q: Can an engaged Bile Thrall make a Purge special attack?
A: No. Purge is a ranged attack.
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Q: Can a Bile Thrall Purge through a Trencher smoke screen?
A: Purge will penetrate a cloud effect, but the Bile Thrall can only perform a Purge if it has a target within LOS.
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Q: What if a model with Stealth is within the area of a Purge?
A: It is automatically hit even if it is further than 5" away from the Bile Thrall. Special rules that cause attacks automatically to hit override special rules that cause them automatically to miss.
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Q: Does Purge trigger Retribution?
A: Yes; however, the Bile Thrall is gone when the damage roll is returned by Retribution. Retribution then expires as normal.
