To unlock this achievement, you must defeat a Shell Horror.
Shell Horrors can be found throughout Aurai, but are more concentrated near Cabal of Wind structures. There is one Cabal of Wind structure in each region, as shown below. They can be difficult creatures to kill if you don’t know their vulnerabilities. You should therefore exploit these by using a weapon or spells which inflict Lightning damage, as all Shell Horrors have a -50% vulnerability to this damage type. You should also ensure that you bring poison antidotes to any fight with a Shell Horror, as they have a chance to inflict Extreme Poison with each attack. Use a weapon with high impact damage to keep the mob knocked down for as long as possible, reducing the possibility of being inflicted by poison.