The was made from a fang of InuYasha's and Sesshomaru's full-blooded yōkai father. In its "docile" form, the sword looks like an ordinary (and rather battered) katana. The sword is enchanted with a barrier that severely burns any full blooded yōkai who might try to wield it, and will not transform at all unless the wielder has the desire to protect humans (though this requirement is only seen once). In its active state, it transforms into an incredibly large and heavy weapon that somewhat resembles a cross between a traditional ōdachi and a falchion. In this form the tsuba (handguard) changes into fur and the rest of blade resembles a very sharp fang. This fur is supposed to be white, similar to Sesshomaru's mokomoko, since it comes from their father who had silverish white fur. In the anime, however, the coloring for it is inconsistent, sometimes white and sometimes brown. Since Tetsusaiga is a sword designed to protect humans, its attacks are extremely deadly to yōkai in particular. It is said that the Tetsusaiga has the power to slay 100 yōkai in one swing by striking using the Wind Scar technique. One notable example of this ability was the effects of the Wind Scar on Sesshomaru, who has shown incredible resistance towards holy and divine attacks, yet was injured severely by the Wind Scar. Furthermore, Tetsusaiga's Bakuryuha technique can only be performed on a yōkai or something with youki energy. Thus, it can be concluded that Tetsusaiga was created to specifically kill yōkai. Non-yōkai with unusual powers, such as Kikyo or some holy men, can sometimes neutralize the sword or its attacks.
Upon InuYasha's father's death, Myoga acted upon his lord's final instructions and hid Tetsusaiga within the tomb of InuYasha's father. The gateway to the tomb was then hidden inside a black pearl that was somehow cast into InuYasha's eye without his knowledge. Upon receiving Tenseiga after his father's death, Sesshomaru began to seek out the Tetsusaiga, and was enraged to discover that it was enchanted against his ability to master it. He has wielded the Tetsusaiga in brief moments, but the barrier around Tetsusaiga rejects him, severely burning his flesh. It is obvious that InuYasha's father intended this from the beginning, so that the brothers could not duel to the death after he was gone: Tenseiga could not kill the living until Tōtōsai reforged it, and it has protected Sesshomaru from the blows of Tetsusaiga on occasion, although he was still weakened considerably for a time, most likely due to it not protecting him until after he'd already been hit by the brute force of the attack. Tetsusaiga's sheath similarly protects InuYasha from attacks by other yōkai swords.