When you visit a graveyard you can find glowing roots called Taint. Collect these and wait until it is close to nightfall. Then, start dropping them on the floor. Back up slightly and wait. As soon as nightfall hits the Taint will spawn Undead Ghouls, Zombies, etc. The number of Taints you have will determine the number of undead that will spawn. Kill them and collect your ingredients. If you kill at least ten Ghouls and collect their brains with alchemy maxed, it is possible to increase your willpower by 18 with 10 Ghouls Brains every night.