Episode 43: "Tetsusaiga Breaks" where Inuyasha confronts Kikyo as to her motivations for giving Naraku Kagome's jewel shards and she explains it is part of her plan to destroy Naraku. Meanwhile, Naraku has unleashed his third incarnation, the telepathic monster Goshinki, who manages to break the Tetsusaiga but unexpectedly cause Inuyasha to transform into a full-blooded demon.
Episode 45: "Sesshomaru Wields Tokijin" where Kaijinbo is destroyed by his own creation - the sword Tokijin - because it is too evil for its maker to wield. Kagura directs Sesshomaru to Tokijin and watches as the demon claims his sword and wields it against his brother. While Inuyasha and his friends escape before they feel the full brunt of Sesshomaru's new weapon, Kagura decides that Sesshomaru may be strong enough to destroy her master.
Episode 51: "Inuyasha's Soul, Devoured" where A moth demon named Gatenmaru traps Inuyasha and Miroku in a venomous cocoon with the intention of taking the Tetsusaiga away. Unfortunately, once separated from the sword that seals his demonic blood, Inuyasha begins to transform.
Episode 52: "The Demon's True Nature" where Inuyasha's demonic transformation gives him great power, but no sense of control over himself as he kills indiscriminately, which Sesshomaru discovers when he arrives to gauge his younger brother's power.
Episode 53: "Father's Old Enemy: Ryukotsusei" where Hoping to find a way to wield Tetsusaiga, Inuyasha goes to Totosai, who instructs him to defeat Ryukotsusei, an enemy who his father could not defeat, in order to gain the strength required to lift his reforged sword. With Ryukotsusei asleep, the task is simple - until Naraku decides to wake the dragon up.
Episode 54: "The Backlash Wave: Tetsusaiga's Ultimate Technique" where Inuyasha fights both his demon transformation and Ryukotsusei and regains his ability to wield the Tetsusaiga, as well as a new technique: the Backlash Wave.
Inuyasha Movie #2: "The Castle Beyond the Looking Glass"
Inuyasha Movie #3: "Swords of the Honorable Ruler"
Note: Not all of these answers may be exactly correct.