From Lexicanum

The Bonding Knife is part of the Tau ritual of the Ta'lissera, in which a unit is bonded as brothers and as a group. They then carry the knife with the unit showing they have bonded, although it is not designed for use in combat. Battlesuits may also bond, although they do not need to carry the knife, instead they may paint a figure of a knife somewhere on the battlesuit.

Ta'lissera

Ta'lissera is a ceremony that may be performed by Tau Fire Warriors commit to support each other even unto death. This closely bonds the unit closely and promotes acts of heroism within a unit.

Sources

• Warhammer 40,000: Wargear

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