Magic Spear of Binding is one of the magical items described in Monster Girl Encyclopedia World Guide - Side II.
Guide Book Entry
As the name implies, it is a magical spear that can be turned into a snake that wraps itself around an opponent. The spear of binding is one of the most famous weapons in Lescatie. These spears are mostly used by the dark knights of the Lamia Squad, led by one of the Seven Princesses of Lescatie, the Echidna Mersé.
During my journey, I heard rumors that among the weapons used by the monsters are "spears that turn into serpents." This magical spear of binding is probably the source of these rumors. Leaving aside the properties of mamono realm silver, such as depriving the enemy of mana, which are common to all monster weapons, the magical capabilities of the spear itself are very simple: with magic it is possible to turn it into a snake. It is not difficult to interrupt this transformation of the spear into a snake that encircles your body. However, if this weapon is used by an experienced dark knight, the threat increases dramatically. If the Knight accumulated mamono mana in the magical spear in peacetime, using the properties of mamono realm silver, then after the transformation of the spear, the snake that will wrap around the opponent's body will also contain the depraved mana of the owner of this weapon. In other words, not only will the binding power increase, but the mamono mana will corrode the person all the time he is in captivity.
If the captured person's mana is corroded by the mamono mana, he will become a mere "prey" that will provide his body to the dark knight, rushing toward him with a specific predatory smile. In addition, magic snakes are used in the "Serpent God Rite" to turn human women into "Lamia" or "Echidnas". Therefore this spear can serve as a substitute for such a ritual: a woman struck by the magic of this weapon will turn into a monster with the lower body of snake.
Magic Spears of Binding are also known for their wide variety of effects - when you fill a spear with mana you can not only increase the binding force, but depending on the quality of mana, some spears turn into huge snakes; others can turn into a snake with poison with a strong aphrodisiac effect, because of which the bite's heat will not disappear from your lower abdomen for a very, very long time; Spears of the most skilled dark knights can take the form of a variety of different snakes.
Caring and diligent training with these weapons is synonymous with honing one's skills to more reliably obtain a husband." Mersé's subordinates seem to polish their magical spears with their tongues and rub their bodies against them gracefully, both day and night, so that the weapons will be more reliably saturated with the lance-owner's mana.[1]