The Triumbric Stones are the three ancient gemstones tied to Gunmar's lifeblood. When brought together, these stones are the only means of wounding and permanently killing Gunmar.
Each Triumbric Stone was created at a transcendent point throughout Gunmar's life: his birth, his first kill, and the battle in which he took control of the Gumm-Gumms. These mysterious stones were hidden for centuries in three remote places and without any record of their whereabouts.
However, a trollish scholar named the Dishonorable Bodus discovered that these stones could be used as a means to create a weapon capable of destroying Gunmar, due to their affiliations with his lifeblood. He then made it his life's work to locate each of these stones. Because of this, Gunmar ordered his Gumm-Gumm Army to assassinate Bodus and all his disciples to prevent anyone discovering his weakness.
Luckily, since Bodus knew that he was being hunted and that he had to keep his discovery of the gemstones a secret from Gunmar and his followers, he was able to leave a hidden message in one of his books, The Final Testament of Bodus, on the whereabouts of each of the the stones. By burning the last surviving copy of his work, it reveals a message about the stones' whereabouts:
Legend of the Triumbric Stones
"In darkest tide, when Daylight darest wane,
Three forces Elemental thou must seek,
Where worthy perish, ye'll prevail in night,
And Eclipse all who quarry with thy might."
The first stone is a fragment of Gunmar's birth, from the first Heartstone which was corrupted by the first wars between trolls and humans. This stone was hidden within Gatto's Keep in the realm of the Volcanic Trolls below Argentina or more specifically, inside the vault contained within Gatto's own stomach. Blinky and Toby were able to recover the stone from the vault and then it was cleaved by Vendel, allowing it to be placed inside the Amulet of Daylight, thus increasing its power and giving Jim the ability to summon two boomerang glaives.
The second stone is a remnant of Gunmar's first kill, where he shattered the Wumpa King. This stone was hidden within the Swamplands in the tribe of the Quagawumps below Florida, which was embedded in the king's crown. Angor Rot ends up killing the current king and steals the stone while losing his Shadow Staff in the process. However, Jim was able to use the Kairosect to retrieve the stone. The stone was then placed in the Amulet, giving Jim the capability to summon a large warrior's shield.
The Eye of Gunmar
The third stone is a commemoration of Gunmar's vulnerability, which is Gunmar's own eye he lost in his battle against Orlagk for control of the Gumm-Gumms. This stone was kept within the Deep Caves in the care of the Krubera; however, the Gumm-Gumms stole it centuries ago and it ended up in Strickler's hands, who kept it hidden for centuries. He finally gave it to Jim so he could defeat Gunmar. After being placed on the Amulet, it allows Jim to summon a helmet that protects him from the effects of Gunmar's Decimaar Blade.