Moo-rderer is an achievement/trophy you can earn in the Witcher 3’s expansion, Hearts of Stone. To get it, you’ll have to engage in some bovine slaying on a mass scale (vegetarians need not apply).
How to get the Moo-rderer achievement
Go to Martin Feuille’s Farmstead, south of Garin estate, and enter the enclosure where the unsuspecting cows are grazing happily.
The cows can be killed with a single hit, regardless of the weapon chosen, and you do not have to fear retribution.
Location of Martin Feuille’s Farmstead
The cows await
Once you’ve bested your bovine foes, meditate for an hour to spawn new cows. Slice, dice, and repeat until the game notifies you that you’ve earned the trophy (after 20 cow kills). You can celebrate by munching on some beef.