Source: Warhammer Armies Project: Unofficial 9th Edition

Multi-Wound Models and Multi-Wound Weapons
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If a unit of creatures with more than 1 Wound on their profile is hit by a weapon that causes multiple wounds, determine how many wounds are caused on each model individually (remember that each model cannot suffer more wounds than it has on its profile). Add up all wounds caused on the unit and then remove the appropriate number of models, noting any spare wounds on the unit. This method is also applied if such a unit is attacked by a spell or weapon that causes a hit on every model in the unit.

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