During my standard event game, my opponent attacked me with three creatures. An unblocked 1/1 creature and a Monastery Swiftspear both with no special effects. I've blocked only the Swiftspear. After blocking, I have passed the priority to the opponent. He used all of his mana except 1 to cast a Tyvar's stand on his 7/3 Emberheart Challenger which was not blocked. With prowess trigger and the spell on the stack, I've responded with a Lightning Strike to kill the Challenger. He responded using his last mana to cast another Tyvar's stand to protect it. With second prowess trigger and the spell on the stack, I've cast a Witchstalker Frenzy to kill the Challanger. The spells resolved, the creature should die but I for an unknown reason I died anyway and my life went from 8 to 0.

I have no idea what the **** happened. The only thing that comes to my mind is a bugged Valiant trigger, that gave the creature indestructible somehow.