When thinking of "strong" characters on Game of Thrones, Samwell Tarly may not immediately come to mind. But he's done something that many of his physically adept comrades can't claim: he's killed a White Walker. In the final season's second episode, as Winterfell prepares for the big fight against the dead, Sam defends his position on the frontlines by listing some of his greater accomplishments, like slaying one of their zombie enemies. Can't remember when and how he did it?
Back in season two when the White Walkers are still a big mystery in Westeros, the Night's Watch go on a mission to investigate them. They stop at Craster's Keep, something of a safe house run by a wildling named Craster and his many daughter-wives. This is where Sam meets and falls for Gilly, who's pregnant with Craster's baby. Jon Snow finds out that Craster's not exactly the best dad, as he's been sacrificing all of his baby boys to the White Walkers.
In season three, during a mutiny led by Night's Watch traitors, Sam and Gilly run away from Craster's Keep with her baby. With their sights set for Castle Black, they stop to rest in a cabin along the way. They end up hearing a bunch of crows squawking like crazy and realise a White Walker has come for them — more specifically, for Gilly's baby, since he skipped the whole sacrifice thing. Sam tries to fight off the White Walker, but it seems impossible until he pulls out a Dragonglass dagger, which he found at the Fist of the First Men in season two. The Night's Watch had encountered White Walkers and wights then, but Sam didn't get in on the fighting action then. This time, however, he manages to stab the White Walker in the back with the dagger. It immediately stops the White Walker, who falls over and completely shatters.
Not only was killing a White Walker a great heroic feat for Sam, who isn't the most adept or interested fighter, but it's the first time anyone on the show discovers that Dragonglass can kill a White Walker. Prior to that, they knew fire could take them down thanks to an encounter Jon had previously. Now that the Night King and his undead army are marching toward Winterfell, many of the soldiers fighting for the living are equipped with Dragonglass weapons. They have Sam to thank for that discovery, and now that he's volunteered to fight as well, maybe he'll even kill more White Walkers.