Excalibur

 is a one of several powerful artefacts that can be used to open gateways to the underworld, or as a weapon.

It was once wielded by fabled English Monarch, King Arthur.

It is a central part to the plot of Tomb Raider: Legend.

Early History
Excalibur and the other swords were forged in ancient times, to be used with the stone daises in order to open portals to the underworld.

Arthurian Times
After the sword in the stone was lost, King Arthur was gifted Excalibur as a replacement sword.

Arthur wielded the sword in battle, upon his death, the sword was broken and the fragments were scattered.

Mid 20th Century
Richard Croft discovered the Ghalali Key on a dig, which he later gifted to his wife Amelia as a broach.

Ameila was wearing the broach when the plane she was on crashed in the Himalayas. She left it in the wreckage of the plane when she took Lara to try and find shelter.

Tomb Raider: Legend
While trying to find out what really happened to her mother, Lara Croft finds out that the events are linked to the Arthurian monomyth. She sets out to find King Arthur's mythical sword. The sword has been split into five pieces which were scattered across the world:

1st Piece - Tiwanaku, Bolivia (brought to Ghana by James Rutland)

In the level Bolivia - Tiwanaku Lara sees James Rutland escaping with one piece of the mythical sword. She later follows him to Amahlin, Ghana, where the Ghalali Key was supposed to be, and takes it from him.

2nd Piece - Tokyo, Japan (in the possession of Takamoto)

The second piece was previously on display in Waseda University, a private university in Tokyo, Japan but was stolen by the Yakuza Clan Boss, Shogo Takamoto.

3rd Piece - Kazakhstan

The third piece is found in a secret laboratory in Kazakhstan.

4th Piece - Cornwall, England

The fourth piece lies with King Arthur in his tomb in Cornwall.

Ghalali Key - Nepal (brought there by her mother, originally located in Ghana)

The Ghalali Key was originally hidden in Ghana, but Lara's father found it, and gave it to his wife as a present. Amelia Croft wore it the day the plane crashed in the Himalayas, and lost it inside, where Lara eventually retrieves it from.

Trivia

 * As of Tomb Raider: Underworld, Lara has two of these swords in her possession.