This achievement requires killing Aknush. Contrary to the above solution, it is not locked to Falon. The way you get this achievement slightly differs based on whether you picked Darius or Falon for the Assassinate Tiberius quest. If you picked Darius, Ishan in Meltwater Basin will give this quest. If you picked Falon, the...head dude I forgot the name of in Red Smoke Camp will give you a quest to do it. Despite who you picked the path is the same after you get the quest. You go to Sinter Tor and fight a few enemies, then you're sent back to Ishan or the Red Smoke Camp guy, and then you go back to Sinter Tor. Beat the boss here, and when you leave you'll encounter Aknush.
In the Sura Wastes is an area called the Ice Caverns, Aknush can be fought for real here. From the cenotaph in Ice Caverns you need to head southeast, when you reach a big room go west and flip the switch. Return to the cenotaph to find a rock, push it all the way west and then north onto the pressure plate to open a bridge, then go north across it to Aknush. Cut him and his demon down and you'll get this achievement.