We Will, In Fact, See It Again

This achievement is most likely the follow-up we all were expecting to We'll Never See That Again, Surely. I haven't done the fight yet but seeing as we will be fighting a "Fleshy Monstrosity" and are going to be returning it's meat to "The Emptiness", I think it's pretty safe to say this achievement is somehow connected to the Sarkareth fight in Aberrus :)

To complete this achievement before the final boss near the teleporting orb is a small puddle that grants a debuff, you must get 10 stacks of the debuff to get a lump of flesh, which gives you an extra action button to throw it and lose some hp. Once you kill the first 3 minibosses Alleria will toss you up in the air and pull you to the center. Walk towards the center and toss the lump of flesh in. Once everyone throws the lump of flesh in a fleshy monstrosity will spawn. (Focus this mob as he has a convert mechanic that makes you dmg those around you.

once you get the "lump of flesh" buff, you cannot use any immunities (i.e. cloak of shadows, divine shield) between when you pick it up and the intermission where you use your extra action button, as you will lose the debuff and therefore not have anything to throw into the black hole.