If Lee Sin is killing Noxus Saboteur (which prevents hidden cards from being revealed) while Lee Sin is also being killed, can the hidden card be used after Saboteur dies but before Lee Sin leaves the battlefield?

Ruling: No, there is no window to use the hidden card. All units with lethal damage are removed at the same step of cleanup, so both Lee Sin and Noxus Saboteur die simultaneously.

Nuances:

• This would be different if there were deathknell triggers involved, which could create a window
• This ruling applies specifically to hidden cards; replacement effects like Sett's ability work differently because they replace the death at the point it would occur rather than activating as a response