Underlust

After years of being sealed underground, monsters began suffering due to infertility, their numbers dwindling. Years later the King and Queen had a stillborn child, Soon a child fell. This child had the soul trait of LUST. They killed everyone who got in their way for their pleasure, Eventually, the human was killed and the soul was captured. After a while, the monsters used the soul to raise their fertility rate. Doctor Gaster started injecting every age with LUST including himself. It was never known whether it increased fertility for them, but every adult monster is always in a state of 'HEAT', needing sexual interaction. Year's later a human falls, wanting to show them what love is, wanting them to understand that is more to love than just LUST.