When Warwick attacks and triggers its ability, can the opponent play an action spell to kill Warwick before its effect resolves, or must they kill it before it moves?

Ruling: When Warwick attacks and its ability triggers, only reaction spells can be played in response. You cannot play action spells before Warwick's effect resolves. You can kill Warwick with a reaction spell (like Hidden Blade played from hidden) before its effect resolves, which will prevent the effect since "here" no longer refers to a battlefield when Warwick is dead.

Sequence:

• Warwick moves to an occupied battlefield
• Warwick's attack trigger goes on the chain
• Only reactions can be played at this point
• If Warwick is killed by a reaction before its effect resolves, the effect does nothing ("here" is no longer at a battlefield)
• If Warwick's effect resolves while it's alive, it kills all damaged units at that battlefield

Nuances:

• It's generally impossible to kill Warwick before it moves if played and immediately moved, since playing a unit doesn't give a reaction window (permanents auto-resolve)
• If the Warwick player combos with Blade Flurry, you can react to Blade Flurry with a reaction damage/kill spell
• Hidden Blade played from hidden (reaction speed) works to kill Warwick; Hidden Blade from hand (action speed) does not
• If Warwick moves to a different location after its trigger, the effect applies to that new location