A mystical sword from the Elder Scrolls fantasy series, and plays a major part in the game Morrowind. It was created by the Dwemer (Dwarf) architect Kagrenac in order to tap the power of the Heart of Lorkhan and create a giant powerful war machine named Numidium in conjunction with the hammer Sunder while wearing the protective gauntlet Wraithguard. At the end of War of the First Council, Kagrenac saw his people defeated by the Chimer and decided the only way for the Dwemer to survive was for him to use the Heart's power to make them immortal, but when he used the tools, his entire race vanished from the known world.
Dagoth Ur of House Ur, ally to Indoril Nerevar during the war, took Sunder and Keening and experimented with them to discover their power. This seemingly drove him insane, and his former allies had to fight him off. The Tribunal (Vivec, Sotha Sil and Almelexia) then inherited Sunder and Keening and used them on the Heart of Lorkhan to make themselves immortal, and give themselves god-like powers.
In order to renew their powers and keep themselves strong, the Tribunal found that they had to make annual trips to Red Mountain (where the Heart of Lorkhan was found). Dagoth Ur attacked Red Mountain, and when Almalexia and Sotha Sil attempted to re-take the Heart, Dagoth Ur was able to seize Sunder and Keening, although Vivec managed to save his two friends.
When the Incarnate (Indoril Nerevar reborn) arrived on the island of Vvardenfell and proved themself to be the Nerevarine, they were charged with the task of reclaiming Sunder and Keening from two towers within Ghostfence, both of which were defended by Dagoth Ur's generals. Once they were reclaimed, the Nerevarine went within the fortress at Red Mountain, and destroyed the Heart of Lorkhan using the tools of Kagrenac, destroying Dagoth Ur and his unfinished creation the second Numidium and making the Tribunal mortal again
Keening was the tool used to create the precise tone from the Heart of Lorkhan in order to get the desired effect. By striking the Heart with Keening 5 times after striking it with Sunder once, the Nerevarine was able to destroy it. It was never revealed how to create any result other than the destruction of the Heart, but presumably it could give immortality and make a person very powerful in terms of the magick they wield.