Over his life Darth Vader killed and stole the lightsabers of many Jedi in many different parts of the Galaxy. But there was one lightsaber that when he wielded, he felt a sense of disgust and hatred… That was the lightsaber of his Master Palpatine.
Sadly for Darth Vader, he was swiftly defeated on the lava banks of Mustafar fight with his Master Obi-Wan, causing him to lose his prized lightsaber. He wouldn’t see this blade again until many years later when he faced down his own son Luke Skywalker, using it against him. This left Darth Vader without a weapon to use in his everyday duties, which included hunting down and killing Jedi surviors of Order 66. As you can imagine this is a big problem.
Vader was deeply hurt by this, as it was a sign of his complete and utter failure and embarrassment on that day. We know in Canon that Vader’s first Sith lightsaber after he killed the Jedi Master Kirak Infil’a and stole his Kyber crystal, before bleeding it to turn it red. But in between the loss of his Jedi saber on Mustafar and that point, he needed another to hunt down Order 66 survivors. For this task, Sidious gifted his apprentice his own backup lightsaber, as all of the other Jedi ones had been burned by Mas Amedda at a public propaganda event. As a result of this, Vader was forced to use this blade for quite some time.
Upon first accepting the blade from his master, Vader actually admired its beautiful and precise craftsmanship, marvelling in its shiny exterior. This was because it was made from the nearly indestructible material known as Phrik, as well as being coated with Electruum, specifically to offend Jedi Masters. Palpatine knew that only Jedi Council members were given the honor and privlege of coating their blades with Electruum, so he went ahead and did it just to taunt the order.
Over time though, things started to change. Vader wasn’t happy with how the saber’s hilt felt in his hands, and on top of that, the blade started rejecting Darth Vader. As we know, kyber crystals are living beings, and this one refused to sing in tune with Vader in the force, making his swings much weaker and less precise. On top of this, Vader noticed that the Saber’s ancient and almost ceremonial design was heavily hindering his combat abilities because it was close to a display piece than an actual fighting blade. He could tell that it wasn’t really intended to be used often, more akin to an artefact or ancient relic.