Soul Snares are your gateway into the Demon Will portion of Blood Magic.
Using the Snare is simple enough - craft a good quantity of them and throw them at hostile mobs. White particle effects will appear around them; then it's time to go in for the kill. On death, they will drop a Demon Will. Before you ask, yes, the Looting enchantment will increase the amount of Will dropped. Once you've gathered a couple, you can get to work on crafting yourself a Sentient Sword and a Tartaric Gem - these will make collecting Demon Will much easier.
Snare on Skeleton
A skeleton with white particles after hit by a snare.