This lightsaber, featuring a red blade, belonged to Darth Plagueis, the Sith Lord. He constructed it during his apprenticeship under Darth Tenebrous.
Following Plagueis's murder of his master on Bal'demnic in 67 BBY, he retained ownership of his lightsaber, even while training Palpatine. When Palpatine executed Pax Teem as revenge for getting the better of him and his master, he utilized Plagueis's weapon. Palpatine observed that the hilt was designed for a larger hand and required him to use both hands for proper use.