I'm confused, so I'm going to leave here several quotes for us to consider.
- Arkat has always had the Unbreakable Sword: Whatever the truth, Arkat possessed a powerful weapon before he left the island [Brithos]: the weapon was called God-Cleaver, reputed to be the Unbreakable Sword. [The Guide to Glorantha, page 128] Arkat’s initiation into the cult of Humakt in 426 was hailed as a victory by the Swords (high priests) of that cult because Arkat’s weapon had always been the Unbreakable Sword, supposedly wrought by Humakt himself. [The Guide to Glorantha, page 129]
- Arkat won the Unbreakable Sword well after the start of the Gbaji wars: Sometimes Arkat was halted [by the Golden Empire]. Then he would stop to HeroQuest, and return with some new and devastating wonder. On one of these HeroQuests he won Humakt's Unbreakable Sword. [Dorastor: Land of Doom, page 9]
- Unbreakable, huh? Well, Arkat, I hope you kept the receipts: The Deceiver was dismembered by Arkat's great sword, which broke from the task. [The Guide to Glorantha, page 386] Numerous ancient magical treasures of the lost Feldichi race and the Bright Empire, including pieces of the Unbreakable Sword (...) lie hidden in Dorastor. [Dorastor: Land of Doom, page 4]
So yeah, when did he win that stupid sword? Why had something so... breakable the name "Unbreakable"? Where is Arkat's Saga when a guy needs it? So many questions! Only in Glorantha, folks.