This achievement/trophy will take some time and a lot of patience. Plan a long route through the regions with humans, and travel along the route making sure you check all major settlements. As you explore the settlements, watch for any human that is glowing red. You will want to charm someone who has at least 95% pure blood. 98-99% is preferred. Once the human is charmed, jump on your horse or use a Dusk Caller to bring them to an empty prison cell.
The goal is to use Irradiant Gruel to boost the prisoner’s blood quality to 100%. Unfortunately, each dose of gruel will bump the quality by 1-2%, but it has a 35% chance to fail and turn the prisoner into a mutant. This is why you want to find someone who is 98-99% pure. Be ready for many failures as you try to get the perfect test subject.
You can make this process slightly easier by changing some server settings: floor the Crafting Cost Multiplier and max out the Crafting Rates. This will make creating and using gruel free, so you won’t need to farm countless materials while trying to find the perfect test subject.