The Acolyte gave us a first look at a lot of firsts in the Star Wars universe, but one of the coolest reveals came in the final episode of the series. In the Season 1 finale of The Acolyte, Osha is confronted with the brutal truth that Sol killed her mother and then lied about it.
Upon hearing this, Osha snaps. She picks up Sol's lightsaber and, rather than striking him with it, she Force-chokes him to death. It's at this point when The Acolyte shows us something we've never seen happen on screen before: lightsaber bleeding.
What is Lightsaber Bleeding and why you've probably never heard of it
If you've never heard of Lightsaber bleeding in Star Wars, don't worry, you're hardly the only one. This is not only a relatively new concept in the universe of Star Wars, but one that you'd have only known about if you've read some of the books or comics.
Before we get into lightsaber bleeding, we must first understand how lightsabers get their color. This comes from something called Kyber Crystals, which Jedi use to construct their lightsabers. A kyber rystal is a rare, Force-attuned cyrstal that grows naturally on various planets across the galaxy. As part of their Jedi training, youngings are sent to Crystal Cave on the ice planet of Ilum to seek out and obtain their own lightsaber crystals. It is these crystals that grant a Jedi their lightsaber hue.
A Sith's red lightsaber came from artificial crystals... until recently...
Until The Acolyte, lightsaber bleeding had never been shown on screen. It wasn't even a concept back when George Lucas was creating the first six movies in the Skywalker Saga. Back when he was writing the movies, the Sith acquired their red-hued lightsabers through a synthetic lighsaber crystal that was artificially created, rahter than naturally formed by geological processes.
According to new lore, Sith now acquire the red lightsaber hue by "bleeding" an already existing kyber crystal. In order to do so, a dark side user must seize a pure crystal from the lightsaber of a Jedi they have vanquished in combat. Then, by infusing it with their pain, anger and hatred, they can bend it to their will. As the crystal becomes corrupted, it aligns with the dark side and starts to turn crimson red.
The concept of lightsaber bleeding didn't actually pop into canon until the 2016 novel Ahsoka, written by E.K. Johnston. But still, it was only first mentioned.
We didn't actually see lightsaber bleeding until the fifth issue of Star Wars: Darth Vader (2017). In this sisue, Vader tracks Jedi Master Kirak Infil'a to the river moon of Al'doleem where he eventually kills the Jedi Master and retrieves his curved-hilt lightsaber.
Vader then returns to Mustafar where he corrupts the kyber cyrstal, teaching it his pain and anger. "Make it bleed," the comic page reads.
From then on, lightsaber bleeding has officially been the canon for a Sith's red lightsaber.