Last Whisper's Unique ability:
Does it work with Sivir's Ricochet?
Does it work against towers?
Google search has left me with conflicting answers and I am not sure what is the most current info.
If an attack does physical damage, it works with Last Whisper. However, NO forms of armor or magic penetration work against towers.
ricochet does not apply on hit effects like black cleaver either (just to clear that up too)
Turrets are only effected by your champion's flat armor penetration, not by items such as Last Whisper.
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