What Gendry's warhammer means to Thrones lore
During Thrones' first season, we’re given expository glimpses into the war that put Robert Baratheon on the Iron Throne. We’re told that the Mad King Aerys burned Eddard Stark’s father and brother, and Prince Rhaegar Targaryen kidnapped Robert’s betrothed true love, Lyanna Stark. Ned Stark found himself the Warden of the North and teamed up with Robert, who he assumed would be his future brother in-law, to ride South, rescue Lyanna, and unseat the Mad King. Robert had to ride his army North from the Stormlands to meet the Stark forces (and those of Jon Arryn of the Vale, the person whose death kicks off the series). Once the rebel forces were united, they brought House Tully into the battle when Ned married Catelyn and Jon Arryn married Lysa. The forces then turned South to unseat the Mad King.
Even when Robert Baratheon was a knight at tourneys, pre-dating the rebellion, he didn’t like to joust -- which might explain his boredom in the first season’s tournament. Instead, Robert liked the melee, an event where fighters compete in combat off a horse. That’s where he could bludgeon his opponents with his warhammer. Robert Baratheon’s warhammer was large and spiked, per George R.R. Martin's books, forged for him by Donal Noye (a now-one-armed Brother of the Night’s Watch in the novels who has not appeared on the show). In the novel Game of Thrones, the warhammer is described as so heavy that Ned Stark could barely lift it -- and this is a guy who fought with Ice, a giant Valyrian Steel broadsword so large it got melted down into two swords for the Lannisters in the premiere episode of Season 4.
Valyrian Steel, a model shop in the business of officially licensed Thrones replicas, gave us a glimpse of Robert’s Warhammer in a limited run of the weapon. Since it was for collectors not battles, it only weighed 10 lbs, but look at how menacing it is: