The sword Tessaiga
Because Inu Yasha and Sesshoumaru constantly fight over the ownership of the Tessaiga, the sword plays a big part in their rivalry. The text underneath explains a bit on the history of the Tessaiga. It was taken from Wikipedia.org.
The Tessaiga was made from a fang of Inuyasha’s and Sesshomaru’s full-blooded yokai father. In its ‘docile’ form, the sword looks like an ordinary (and rather battered) katana. The sword is enchanted so that no one, human or yokai, may wield it unless they have a desire to protect humans. In its active state, it transforms into a weapon roughly the size of a car bumper that somewhat resembles a falchion, and has the power to slay 100 yokai in one swing by striking the Wind Scar. In this form the 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 sama, since it comes from their father who had silverish white fur. In the anime, however, the coloring for it is inconsistent as sometimes its white and sometimes its brown. Tessaiga is a sword designed to protect humans, therefore its attacks are extremely deadly to yokai in particular. Throughout the anime and manga, Tessaiga’s attacks are shown to be most effective against yokai. One notable example of this ability was the effects of the Wind Scar on Sesshomaru. Sesshomaru has shown incredible resistance towards holy and divine attacks, yet he was injured seriously by the Wind Scar and took days to recover, even though his own sword Tenseiga was shielding him from the blast. Also, Tessaiga’s Bakuryuha technique can only be performed on a yokai or something with demonic energy (youki). Thus, it can be concluded that Tessaiga was created to specifically kill yokai. Non-yokai 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 Tessaiga 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 Tessaiga, and was enraged to discover that it was enchanted against his ability to master it. He has wielded the Tessaiga in brief moments, usually when battling an out-of-control Inuyasha, but the barrier around Tessaiga which rejects him burns his flesh and leaves him weakened afterwards. 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 Totosai reforged it, and it has protected Sesshomaru from the blows of Tessaiga on occasion, although he was still weakened considerably for a time. Tessaiga’s sheath similarly protects Inuyasha from attacks by other demonic swords, and is more powerful than any other sword except Tenseiga (since they cancel each other out) and presumably so’unga.
In a confrontation with Goshinki, an incarnation of Naraku, the Tessaiga was broken in half. The sword’s creator Totosai was able to repair the Tessaiga, but in doing so he used one of Inuyasha’s fangs. The result is that Tessaiga now became dependent on the strength of InuYasha himself – making the weapon heavier and more difficult for Inuyasha to wield. Totosai told him he would have to equal or exceed his father in order to make the sword lighter. After much training and a battle with the yokai Ryuukotsusei (whom Inuyasha’s father battled to a standstill shortly before his death), InuYasha learns how to use a greater power created from the convergence of the Wind Scar (Kaze no Kizu) and attacks from a powerful opponent, known as Backlash Wave (Bakuryuha). Tessaiga can also absorb magical abilities from the blood of a creature that possesses them, for example, barrier shattering (from Shiori’s barrier orb) and Kongousouha (Adamant Barrage, from Hosenki (Housenki). It is however, interesting to note that he has not ever obtained an upgrade in the fashion that he expected to. The power has only been able to be absorbed after Inuyasha has shown some sort of emotional maturity, morality or personal growth. When Tessaiga became heavy, he ended up losing it when he fought with Ryuukotsusei. He transformed to his full yokai form, and started beating Ryuukotsusei but after being smashed into a cliff, Inuyasha recovered enough of his senses to find and take Tessaiga. Upon taking hold of the sword, it became feather light to him and he was able to use the Kaze no Kizu anytime, along with the Bakuryuha. When he attempted to cut the barrier orb, he could not cut it due to a barrier around the orb. However, when he felt compassion for Shiori and protected her from the orb’s power, he was able to obtain the orb’s power without ever having cut it. The same thing happened with Hosenki. When he attempted to cut Hosenki, he didn’t even seem to scratch him, and instead ended up being peppered with diamond shrapnel that stuck in the wounds like glass. However, he abandoned his attempt to cut Hosenki when he saw that his friends were in danger, and consequently was able to use the Kongosouha technique although the fracture in Hosenki had not yet appeared. As a matter of fact, it appears that the only upgrade that has been directly absorbed by Tessaiga itself by destroying the source of the power was the ability in the next paragraph, which may help to explain the numerous problems he has had using said ability.
Recently, Inuyasha shattered a sword made from the scales of a great dragon, which gained power by absorbing the youki of its opponents. In doing so, he gained the Dragon-Scaled Tessaiga, but until he used it to absorb holy power to purify the incoming youki, he risked great harm to himself in using it. After being dressed-down by his older brother, he went to Totosai, who in turn sent him to Yourei-Taisei, who created a phantom world in which Inuyasha was forced to expand his own ability to sense youki. Doing so allowed him to sense Youketsu, swirling pools of youki that surround yokai. With the Dragon-scaled Tessaiga, he can now break youketsu (although he initially does it by hand, this is in the illusion yourei-taisei created, and so he may not be able to repeat this or may simply not be able to destroy a powerful yokai that way), and in doing so cripple or even destroy opponents. This may be a more efficient use of the Dragon-scaled Tessaiga, as Totosai posits that Tessaiga is “a sword for cutting.” In other words, it is meant to defeat physically, and claim any powers after the fact, not suck them out as Inuyasha had been doing. To do so was holding Inuyasha back from further mastery of the Tessaiga. Also, during the fight to obtain the Dragon-Scale power, Inuyasha proclaimed that Tessaiga could not “die”/break while he himself was still alive, and that he had some sort of spiritual connection to the sword, going so far as to say that they “are as one” (both probably because the sword is connected to his own yokai energy as a result of being forged the second time with one of his own fangs).
