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 the dwarven capital. 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 Proudmoore from Baine before the fall of Theramore, as the tauren did not feel worthy enough to wield it. It is currently in possession of Anduin Wrynn.
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.
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. When Moira Thaurissan held the city hostage with her Dark Iron forces, Anduin hid the weapon, believing Moira would not appreciate that he had been given a family heirloom. When he later escaped by hearthstone to Theramore Isle, he took the weapon with him, and while meeting Baine, was inspired to give it to the tauren after checking that the mace would accept him as it had Anduin.
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 took it back. Baine did not feel he was worthy of holding the weapon.
When Jaina returned to the ruined 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 in the Isle of Thunder, where the Wrynns stayed for the duration of the trial.
When Garrosh Hellscream escaped, King 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. What happened with the alternate weapon is unclear, but it is likely that it was returned to King Anduin's timeline.
Anduin had the mace by his side during the ceremony at Lion's Rest in Stormwind City after the Argus Campaign. When not in use, Anduin's manservant — Wyll Benton — would mount Fearbreaker on a hook on Anduin's bedroom wall so that the king could easily access it just in case.
In War Crimes, it was not clearly stated if the Fearbreaker seen in King Anduin's hands was from his alternate timeline or from the main timeline. If the latter, the weapon shouldn't have been in the temple as weapons were forbidden during the trial and the combatants were given weapons by the pandaren when the chaos erupted. When King Anduin died and returned to his own timeline, the weapon did not vanish with him but was instead taken up by Prince Anduin. However, it's possible that it vanished back to its own timeline at a later point.