Summary
- Jedi Master Yaddle's death at the hands of Count Dooku likely served as a critical Sith ritual, marking his full descent to the dark side.
- Dooku's red lightsaber in the Clone Wars may have been powered by Yaddle's kyber crystal, fulfilling the ancient Sith ritual of using the crystal of a fallen Jedi.
- Yaddle's legacy can be seen every time Dooku ignites his lightsaber with the high probability that he took her kyber crystal to create his Sith blade. This dark explanation adds to Dooku's dark transformation from a Jedi to a Sith Lord.
Jedi Master Yaddle was likely given a very dark Star Wars legacy heading into the Clone Wars. As seen in the Tales of the Jedi anthology series, Count Dooku's final days with the Jedi Order were revealed. This included when his path darkly intersected with Master Yaddle of the Jedi Council, resulting in her death. As a result, it stands to reason that Yaddle was memorialized by the Sith in the darkest way possible.
Of the six-episode Tales of the Jedi anthology, three are focused on Dooku before he truly became the new Sith apprentice of Darth Sidious and led the Separatists during the Clone Wars. This included "The Sith Lord", taking place during and just after the events of The Phantom Menace. In the wake of Qui-Gon Jinn's death, Jedi Master Yaddle followed Dooku, Jinn's former master. It was here that Yaddle discovered Dooku's secret alliance with Palpatine and his true identity as Darth Sidious. Yaddle was subsequently killed, and her death likely served as a critical Sith ritual for Dooku, marking his true corruption to the dark side.
Your browser does not support the video tag. Sith Blades Are Powered By The Kyber Crystal Of Their First Jedi Kill It's A Dark Initiation Ritual 
In the Star Wars canon, would-be Sith apprentices would prove themselves to their masters by killing a Jedi. The kyber crystal powering the fallen Jedi's lightsaber was then taken and bled using the dark side of the Force, becoming the kyber for the new Sith's red-bladed lightsaber. This is a traditional ritual worth considering when looking at Tales of the Jedi. In the episode, Palpatine simply watches Yaddle's duel with Dooku as much as he can, and he notably allows Dooku to strike the first blow. This is likely in accordance with this ancient Sith practice.
Dooku's defeat and murder of Yaddle cemented his corruption to the dark side. Likewise, it also proved his loyalty to Sidious despite his grief over Jinn's death. Having proved himself to Sidious by killing a Jedi, it's therefore likely Dooku's red blade in the Clone Wars was powered by Yaddle's kyber crystal as well, fulfilling the aforementioned Sith ritual.
