This Best Served Cold achievement is likely to be obtained naturally as you play through the main campaign. The business of forging a powerful clan and kingdom in Calradia is a bloody one, and the chances of one of your clan members meeting a grizzly end at the hands of another lord are high. All you need to do to ensure that this achievement is completed is keep track of any death involving one of your clan members, take note of who executed them and then begin the process of hunting them down. If you defeat the lord in battle, you will have the option to release them, imprison them, or execute them at the end. The choice is clear to obtain this achievement.