The Bonding Knife is a ceremonial knife that serves as part of the Tau ritual of the Ta'lissera, in which a group is bonded together through ritualistic cutting of their flesh. Following their ritual bonding the knife is then carried into battle by the team leader, although it is not designed for use in actual combat, and serves as a reminder of the sacred bond the group shares. Fire Warriors who serve as part of a Battlesuit team may also undergo the process, although instead of carrying the actual knife into battle the team leader may instead paint a knife design onto his armour.
The Ta'lissera is a Tau ceremony which has been translated by Imperial xenolexicons as "communion" or "marriage." Tau who take part in the ceremony pledge support and community to one another and may address each other by their individual names. The bond that these Tau swear to each other are much admired and seen as embodying the best aspects of the Greater Good. While the bond may be performed by any group of Tau, it is most commonly Fire Caste warriors or Earth Caste work teams who undergo this ultimate expression of respect for one another.