Demonic Will
From Feed The Beast Wiki
Demonic will can be in 2 forms. Raw demonic will and stored demonic will.
To get raw Demonic Will use Rudimentary Snare's on a hostile enemy till white particle effects appear or use a Sentient Sword. Kill the enemy and you will receive Demonic Will.
To get stored Demonic Will you have to hold a Tartaric Gem while killing an enemy with