Thor's Hammer

This Warhammer is also called the Mjolnir, it was a powerful weapon of Thor which he used to win great victories.

It appears in St. Francis Folly from Tomb Raider I and is represented as a giant hammer in the Room of Thor. It is unknown if it can be used to open a portal to Avalon, but as Lara states in the latest Underworld gameplay trailer, "I need Thor's Belt to get his hammer; and I need the hammer to kill a God".

Meaning, that because Jacqueline Natla has returned in Underworld, Lara may be referring to Natla as the god who is immortal, and must use the hammer to destroy her once and for all. She may also be relating possibly to the god of the Underworld, hence the name Tomb Raider Underworld. It is theorized that Thor's Hammer is in Underworld because it relates to Jacqueline Natla - hence the fact that Lara encountered it in the Room of Thor in St. Francis Folly.