This achievement is fairly simple. If you want to become the Butterlord, you need to loot or purchase at least 100 Butter and have them in your inventory at the same time. It won’t count if you have 50 Butter one day, then 50 the next, since your troops will gradually consume this. It has to be 100 Butter at the same time. Butter is quite cheap in most places throughout Calradia, but you should also keep in mind that the more you carry, the greater your carry weight will be. You’ll need to have a Mule or two to increase your carry weight capacity, but all of this comes quite naturally as you play through the campaign.