Fearbreaker is a mace that was given to Prince Anduin Wrynn of Stormwind by King Magni Bronzebeard of Ironforge during the prince's visit to that city. However, Anduin would eventually give the weapon as a gift to Baine Bloodhoof to assist him in reclaiming Thunder Bluff from Magatha Grimtotem, but it was returned to Jaina from Baine before the fall of Theramore, as Baine did not feel worthy enough to wield it.
Fearbreaker was a family heirloom of the Bronzebeard clan, and had been passed down in Magni's family for many hundreds of years. It had been used in both Azeroth and in Outland. As Magni related it to Anduin, "it's known the taste o' blood and in certain hands has been known to also stanch blood", which implies that the weapon is imbued with healing powers. Indeed, when Anduin (and later Baine) took up the weapon, the Light seemed to flow through them from the weapon, showing that the weapon "liked" them.
Baine wielded the mace when he was attacking quilboars and rescuing Garrosh. It shone brightly like the Light.
Some time after Deathwing was killed, the Horde led an attack on Northwatch Hold and from there planned on attacking Theramore Isle. Baine sent Perith Stormhoof to warn Jaina Proudmoore in advance, and also had Perith deliver Fearbreaker with the request that Anduin takes it back. Baine did not feel he was worthy of holding the weapon.
When Jaina returned to Theramore for the last time, she found the mace, undamaged. She took it with her when she left. During the trial of Garrosh Hellscream, Anduin was seen with the mace in the Violet Rise where the Wrynns stayed for the duration of the trial.
When Garrosh Hellscream escaped, Anduin Wrynn from an alternate timeline appeared in the Temple of the White Tiger, clutching Fearbreaker in his hands. The prince took the glowing mace when the king was overwhelmed, and began fighting hunter Vol'jin from another timeline. Enraged Vol'jin threw his glaive at the prince but he quickly drew the mace, knocking the tossed weapon aside. After the king died, the prince took the weapon and continued fighting and defending himself.
- Gleaming as bright as the day it was made although it was quite old. The head was silver, wrapped in intersecting bands of gold that had runes etched into it. Small gems dotted it here and there. It was altogether a thing of beauty and power.
In the book it wasn't clearly stated if the Fearbreaker seen in the king's hands was from his alternate timeline or the main universe. The weapon shouldn't have been in the Temple as weapons were forbidden and the combatants were given weapons by the pandaren when the chaos erupted. When the King died and vanished into his world, the weapon didn't vanished but the prince took it. It could vanish later.