Tenchi-ken (天地剣, Sword of Heaven and Earth), also known as "Sword Tenchi," has appeared various times in the Tenchi Muyo! metaseries.
In Tenchi Muyo! Ryo-Ohki, the official continuity, Tenchi-ken, or sword Tenchi, is a Master Key for Funaho, a first-generation Royal Tree of the planet Jurai who is bonded to Prince Yosho. The sword was carved from a branch of the mother of Royal Trees, Tsunami and crystallized by its sap. Tenchi-ken is a very powerful master key and has the ability to control most royal tree ships and call upon the power of the Light Hawk Wings to form a lightsaber-like blade.
When Prince Yosho trapped the space pirate Ryoko in the cave near Masaki Shrine, he stole her three gems, created by the Chousin goddess Washu, and placed them on the pommel of the sword. They gave power to Funaho allowing Yosho to retain his youth (though to others, he appears as an elderly man). The sword can only be used by someone of noble Juraian blood and used to be Tenchi's most powerful weapon until his ability to generate Light Hawk Wings surfaced. Background material reveal that the energy blade is made of the same energy as a Lighthawk Wing, but is much less powerful.
By the time of OVA 5, Tenchi no longer uses Tenchi-ken in battle, instead summoning his "Light Hawk Sword", a blade he forms using the energy from one of his Light Hawk Wings. This blade has all of the abilities of the Tenchi-ken, in addition to using Tenchi's matter-alteration and teleportation powers. As for the Tenchi-ken, it is not seen again, possibly returned to a safe place at or near the Masaki Shrine.
In the Universe continuity, Tenchi-ken is only seen toward the end of the series. Here, the sword is merely a weapon from Jurai and there are no intelligent space trees. In addition, the television version of Kagato has a similar sword of his own which resembled a dark bastard sword, as opposed to Tenchi-ken's tsurugi-like appearance.
Tenchi in Tokyo
In the second TV series, Tenchi in Tokyo, Tenchi-ken's appearance and background are different. Tenchi has no connections to Jurai in this continuity, and the sword is created from seven gems which he inherited from his late mother. Tenchi used the sword to defeat a monstrous Ryoko when she and the other six girls crash-landed at his home. Ryoko separated the gems to prevent the sword from being used against her again. Each of the six girls took one of the gems as a keepsake. Because of this, the main antagonist, Yugi attempts to split up the girls so that the crystals will be scattered, weakening them and not allowing Tenchi to form the sword. This fails as the sword's crystals are reunited to be used to defeat Yugi.