Bloody Mess

Enter Ny'alotha, the Waking City on Normal difficulty or higher and engage Il'gynoth, Corruption Reborn.

Defeat Il'gynoth and destroy an organ at the edge of the room to advance the fight. Repeat this process three times total — the Bloods of Ny'alotha (Cursed Blood) do not begin spawning rapidly until Il'gynoth has been killed three times and three organs have been destroyed.

During the intermission phase after the third kill, allow the organs along the sides of the room to cast — each cast spawns 3 Bloods of Ny'alotha. Kite the bloods around the edge of the room in a circle until you have at least 12 stacked on you. The intermission phase is recommended because the boss and beams are absent, reducing incoming damage. Note that bloods gain Blood Frenzy if left alive too long, which prevents crowd control, so do not delay once you have enough stacked.

Once you have 12 or more Bloods stacked together, use your strongest area-of-effect abilities and burst cooldowns to kill them all within 3 seconds. Killing at least 10 within that window completes the achievement requirement.

After the bloods are dead, destroy the remaining organ to continue the fight, then defeat Il'gynoth a final time to complete the encounter. If you die after killing the bloods, do not release until the achievement registers, as releasing during that delay can prevent it from being awarded.