The description of the achievement is self explanatory, now to figure out who you can spare/recruit. At the end of the “Troll Trouble” questline, you will fight agains Tartuk and Hargulka: you can kill one and have the chance to spare the other (focus the one you want to kill, the other one will ask for mercy). At the end of “The Varnhold Vanishing”, Vordakai can become an advisor by sacrificing Maegar, but evil alignment is required. Armag is an option as well, become his blood brother at the end of “The Twice Born Warlord”: to do so you need to get to him before he is driven mad by the sword (meaning, don't go for “Betrayer's Flight” first, go straight for “Hour of Rage” and don't lose time roaming around). An unromanced Nyrissa is also an option, you can strike a deal with her at the end of the House at the End of Time in Chapter 7.
Credit to Starsong for their
guide.