According to The Silmarillion, Anglachel (Iron of the Flaming Star in Sindarin) was one of the two swords forged by Eöl the Dark Elf out of a black iron meteorite. It is best noted to be made from a flaming star, which could be interpreted as a meteorite, or other phenomenon, natural or supernatural. As the stars of Middle Earth are explicitly made from the dew of Telperion, it could not literally be a star. It is said to be able to cleave any iron from within the earth. Anglachel has a will of its own and speaks at least once to Túrin.
There is malice in this sword. The dark heart of the smith still dwells in it. It will not love the hand it serves, neither will it abide with you long.
The sword is said to have sung with gladness when Beleg unsheathed it to kill orcs and other servants of Morgoth.
After Túrin was captured by Orcs at Amon Rûdh, Beleg pursued the Orcs to free Túrin. He slew their guards and slipped into the camp, but as he was cutting Túrin's fetters, the sword nicked Túrin, waking him. In the darkness, Túrin assumed it was Orcs come back to torture him. He seized Anglachel, and slew Beleg with it. When he realised his mistake, he mourned long over the death of his friend.
Túrin and Gwindor then travelled together to Nargothrond where the sword was reforged by the expert smiths and renamed Gurthang (Iron of Death in Sindarin). Its edges shone with a pale fire. The Elves came to call Túrin Mormegil (Black Sword in Sindarin). With it, he led the Elves of Nargothrond in many battles, driving the Orcs from the lands. Túrin also referred to it as the Black Thorn of Brethil.
Much later, after the slaying of Glaurung, when Túrin discovered by means of Brandir that his wife Nienor was in fact his sister and that she had killed herself, he slew Brandir in anger with it, and then in despair he fell upon his sword, asking it to take his own life:
"Hail Gurthang! No lord or loyalty dost thou know, save the hand that wieldeth thee. From no blood wilt thou shrink. Wilt thou therefore take Túrin Turambar, wilt thou slay me swiftly?"
And from the blade rang a cold voice in answer: "Yes, I will drink thy blood gladly, that so I may forget the blood of Beleg my master, and the blood of Brandir slain unjustly. I will slay thee swiftly.
It was prophesied by Mandos that in the Final Battle for Earth at the end of time, Gurthang would again be taken up by Túrin and would deliver the final blow against Morgoth, defeating evil forever.
Another parallel is the tale of Sir Balin in the Arthurian Legend, who with an accursed sword, unintentionally causes misery wherever he goes, until eventually he unknowingly kills his own brother.