The Player's Handbook states that when a Druid in Wild Shape loses all of its hit points, it reverts to Druid form. What would happen if an enemy Wizard casted Power Word Kill on the Wild Shape Druid? Do you use the Druid's hp or the Wild Shape's hp to determine if the spell can work? And if it does, is the Druid kill outright, or does it just revert back to Druid form?