1-3 - Burrow and Hide: The unit immediately Goes to Ground. Units that contain at least one model with the Fearless special rule treat this result as Prowl (below), instead.
4-5 - Prowl: In the Shooting phase, the unit cannot Run and must instead shoot at the closest enemy unit that is within range and line of sight of at least one model in the Tyranid unit. If there is no viable target, the Tyranid unit can do nothing during the Shooting phase. The unit cannot charge in the Assault phase.
6 - Destroy: This follows all the rules for Prowl (above). In addition, the unit gains the Preferred Enemy special rule.
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