Blade/Iceman Indestructible/Unstoppable?

Just fought blade and iceman in arena and both recieved an indestructible and unstoppable buff at 1% health. Been a little while since I played but is this one of their abilities, a synergy, or a mastery? Or what is it? And I do mean the recieved the actual buffs for indestructible and unstoppable
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And if you’re looking for a good offensive team, add Angela and Hela with Heimdall. Your last other attacker gets 100% Stay Alive chance while regen'ing to 20% upon knockout, in addition to separate instance of Heimdall keeping that other attacker alive at 1% as well. And Hela has her own separate instance of Stay Alive too (3x “cheat deaths” if she is last attacker used).