Special event or what.

[Elric of Melnibone]

I did the walking monster, enter the portal, inside one there was a never ending? waves of monster with a bar saying number of coffins of hate. Never saw this before, at Torment I level. I did 12 waves.

[Evabant]

it's a glitched event. There should be a timer that counts down and you get a chest for each time you filled the bar. First time I got it, I had the same thing happen as you and just had to walk out of it.