Lust is one of the Seven Deadly Sins from Abrahamic Lore, which is directly condemned in the Sixth and Ninth Commandments. Like all of the capital sins, lust is considered to be a gateway to other forms of immorality. Lust is a form of intemperance which is directed towards sexual pleasure.
While lust can be a physical form of perverted sex (such as when sex is not open to procreation or is not in proper marital unity), it can also be in impure and excessive desires for said pleasure. It is related to other sins such as gluttony, among other forms of abusing earthly pleasure. In Buddhism, one of the Ten Precepts used to train novice monks tells them to refrain from lust. In Hinduism, Krishna proclaims lust to be one of the sins that is a gateway to hell. The demon of lust is Asmodeus and the corresponding virtue is Chastity.
Lust's Spell Card can't be acquired through normal means, only via the use of cheat codes. It is however usable in fusion and possess inheritable skills.