Even after nerfing him, with 464 million hit points and an extra third phase, Hellfire Citadel's final boss Archimonde on Mythic difficulty remains a tough challenge for guilds. And sometimes it creates truly unlucky moments, like this attempt from a German guild.
They were really close to killing Archimonde but ended up dead against a boss with only 1 HP left. Ouch. With everyone in the raid doing thousands of DPS, this is just not right.
The video is in German, but you can hear the moment when they think they killed him, but actually didn't. And it's a custom UI so the red HP bar you're looking for is in the middle bottom part.
The game's mechanics prevent bosses dying during certain casts, and as you can see Archimonde starts casting Seething Corruption basically at the same time when the killing blow should happen. If they'd been a tiny bit faster they could have succeeded.
It's a happy ending though. Instead of uninstalling the game, cancelling their subscriptions, disbanding the guild and deleting their characters, they killed the boss later that day.