Ruling on revealing hidden units
I have a question regarding revealing hidden units like Evelynn or Windsinger. My understanding was that if I have a unit at a bf and a hidden unit, if my other unit dies, I’m allowed to reveal the hidden one and not lose control of the bf. Today in a game at my local, my opponent moved in with a unit equipped with Forgefire Cape. I had enough might to kill it but all my units also died in the combat. I wanted to reveal my Evelynn afterward to keep the bf, but my opponent said I can’t do that and my Evelynn also dies during combat because of Forgefire Cape.
So when am I allowed to keep a bf like this with a hidden unit? Is it only if my other units die to spells and abilities and not during a combat showdown?