I sent you a screenshot and recreation steps. I was way to hyped trying to find this, so my sentence structure may suck.
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I used Anointed One with more than enough damage to fully kill a Sea Serpent.
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I had 3 skull matches (I think all 3 were 3 skulls only.) set up for when I would trigger Keeper of Souls.
If Agile triggered, the unit went into the un-death mode. Agile won’t always trigger, so you won’t always see it.
I hope this helps track it down.