Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)

Accusation
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After deployment, but before the first turn begins, select a single model in your opponent's army to accuse of heresy. This is the Witch Hunter's primary quarry. The Witch Hunter may re-roll all failed To Hit against his primary quarry. Every hit he inflicts on that model with close combat or shooting attacks also has the Killing Blow special rule. Finally, a Witch Hunter may also choose to shoot at the primary quarry as if he had the Sniper special rule.