-How he got it

--Teleportation
When Sesshoumaru was hit by the Kaze no Kizu in a battle with Inuyasha, Tenseiga surrounded him with a black light to protect him from the blast and transported his body out of harm's way. It seems to be a built in defense mechanism, because it acted of it's own accord.

--Communication
Tenseiga pulsates and/or rattles in it's sheath frequently when it wants Sesshoumaru to pay attention to it. Generally it wants him to go see something or be used, which Sesshoumaru does.

--Resurrection
Tenseiga's obvious use. Though the blade cannot cut anything alive, it can be used to cut through the messengers from the other world that come to take the souls of the deceased. When these figures are 'killed' in a sense, the person they came for comes back to life. The Tenseiga has limits, however - it can only be used on someone that is actually dead, and there has to be a body.

--Souryuuha
--(Movie only)
This attack was created for the third movie. A blast of light/energy is sent at the target. Souryuuha is powerful enough to deflect Tessaiga's Bakuryuuha, which Sesshoumaru combines it with at the end of the movie to defeat Sou'unga.

--‘100 with one swing’
Though it has not actually been shown, Toutousai told the Inuyasha-tachi that Tenseiga has the ability to save a hundred souls with one stroke, which would counterbalance Tessaiga's power to destroy a hundred with one stroke.