Caution: spoilers ahead for The Book of Boba Fett
Why does Din Djarin's Darksaber (and it definitely is Din Djarin's Darksaber now) get heavier during combat in The Book of Boba Fett? Introduced to Star Wars mythology by Star Wars: The Clone Wars, the Darksaber is the fabled weapon of ancient Mandalorian Jedi Tarre Vizsla, but holding the blade has since come to denote Mandalore's true ruler. Disney+'s The Mandalorian finally debuted the Darksaber in live-action, and by the end of season 2, Din Djarin himself was in possession of the sword having bested its previous owner, Moff Gideon, in single combat.
Djarin makes his unexpected return in The Book of Boba Fett episode 5, which interrupts its usual programming to bring Star Wars fans a surprise installment of The Mandalorian. Picking up with Din Djarin following his Luke Skywalker encounter in The Mandalorian season 2's finale, the bounty hunter finds The Armorer, looking to commission a special present for a certain pint-sized pea green Jedi. The two Mandalorians discuss the history and perils of the Darksaber Din Djarin has now acquired, but he complains how the blade seems to get "heavier" with each and every movement. The Armorer accuses her client of "fighting" the Darksaber rather than fighting with it, but a fuller explanation for the sword's phantom weightiness can be found elsewhere in Star Wars canon.
The appropriately-titled Star Wars Rebels season 3 episode "Trials of the Darksaber" shows Sabine Wren training with this tricky titular weapon for the very first time, while Kanan Jarrus acts as her instructor. Upon igniting, Sabine remarks on how the Darksaber feels "heavier than [she] thought," and Jarrus duly explains why. According to the runaway Jedi, the Darksaber is considerably harder to wield than your average lightsaber. Jarrus says, "Energy constantly flows through the crystal. You're not fighting with a simple blade as much as you are directing a current of power. Your thoughts, your actions, they become energy. They flow through the crystal as well, and become a part of the blade." In other words, someone who lacks control and acceptance over their thoughts and feelings - or those with an undisciplined fighting style - will find the Darksaber heavier to hold, as all of that inner baggage channels into the blade, weighing it down.
Din Djarin is a man enduring great turmoil in The Book of Boba Fett. He recently gave up Grogu to Luke Skywalker, learned how his entire Mandalorian creed is actually the doctrine of an extremist cult, accidentally inherited the leadership of his people, and to top it all, his ship was destroyed by the Empire. Not to mention, Din Djarin is more accustomed to blasters and explosives than the more civilized world of lightsaber forms (which Kanan Jarrus made Sabine learn before her Darksaber training began). Mando's swordsmanship struggles are hardly surprising, and the blade's heaviness is to be expected. The longer he uses it, the more of Djarin's energy is flowing into the Darksaber's crystal.
Fortunately, Din Djarin can become a Darksaber expert soon enough. Training with Kanan Jarrus during Star Wars Rebels, Sabine Wren faces her fear and anger over the fate of Mandalore, affirming a determination to tame the black blade so she can protect her friends and family. As the sparring session continues, Sabine eventually acknowledges, "The blade feels lighter," which Jarrus attributes to establishing a "connection" between weapon and owner. If Din Djarin is to make full use of his new toy in The Book of Boba Fett and The Mandalorian season 3, he must forge a similar bond, channeling those emotional energies in the right way, rather than letting them pull down his Darksaber as they flow through its crystal. Maybe Din Djarin also needs a clearer picture of what he's fighting for now that Grogu isn't in his care.
The Book of Boba Fett streams Wednesdays on Disney+.