Throughout the fight, Krosus casts puddles of Burning Pitch on the ground, which will spawn Burning Ember adds if no-one stands in them. In addition to this, the boss will slam the bridge that the raid stands on throughout the encounter; on the third slam each time, a forward section of the bridge will collapse into the surrounding water. If any Burning Pitch adds (or players) are on this section of that bridge at this time, they will fall into the surrounding water and die.
To obtain the achievement, simply ensure that at least 15 Burning Ember adds are "quenched" by falling off the bridge during the fight, then defeat Krosus and the achievement is awarded.
- Patch 7.0.3 (2016-07-19): Added.