The Lust Spear is part of the Lust SeriesCurse Series claimed by Motoyasu somewhere around 『Chapter 165』 of the web novel. Motoyasu became extremely depressed after Elena, Witch, and Woman 1 abandoned him. Little is known of what he did during the intermission between the Spirit Turtle incident and when Naofumi found him, but when he is found, Filo attempts to cheer him up leading him to become mind broken. The Lust Spear IV is revealed in 『Chapter 209』 after Motoyasu fails to win a race against Naofumi when Filo is the prize. Its special effect, 「Temptation」, lends power to those overcome by lust. There doesn't seem to be a repercussion for this curse series though it is speculated to be something along the lines of impotence or the inability to feel euphoria. Although Motoyasu sees women as pigs and can't understand their words, it was unlikely caused by the curse due to the fact that he started calling women pigs and ignoring them at the bar after falling for Filo.
Lust Spear IV 『c209』
Explosion of emotions equivalent to the destruction of one's heart