Riverendare, Warrior is not working if opposing hero is immune, unlike Mecha’thun, the 3 cards that you get from him do say destroy enemy hero blah blah, so fix that bug !
Do you have a replay? I would be interested to see this
Unfortunately no 
but I was playing against Paladin.
I tested it with a friendly game in wild, and my Rivendare still destroyed a Paladin who had played Time Out! and was immune. Maybe something else prevented the deathrattle from going off
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