He was portrayed by Joseph Gatt, who also played Neils Skellig in Dumbo.
When Thor prepares to ascend to the throne of Asgard, he was interrupted when Frost Giants attempt to retrieve the Casket. Against the orders of his father Odin, Thor travels to Jotunheim to confront Laufey, accompanied by his brother Loki, childhood friend Sif and the Warriors Three (Volstagg, Fandral, and Hogun).
Having expected their arrival, Laufey and his Frost Giants (including Grundroth) gave them an unwelcome visit, but gave them a chance to leave. At first Thor and his friends are about to leave, but when Hailstrum taunts Thor by calling him a 'little princess', this provoked an angry Thor to use his hammer to smack Hailstrum in the head, crushing his skull instantly. This event is what led to brief fight against the Frost Giants and the fracture of the truce between the Asgardians and the Frost Giants. During the fight, Grundroth tried to attack Thor, but the latter used his hammer to send Grundroth flying against a frozen mountain. He then rose to fight Hogun with two Ice Daggers until the battle was stopped by the arriving Odin.
This event is what led to the fracture of the truce between the Asgardians and the Frost Giants, right before Thor was exiled to Earth as punishment for his actions. Laufey later learned that it was Loki (who turns out to be Laufey's son) who let the Frost Giants attack Asgard as he was jealous of Thor of earning the throne. As Odin was put to sleep, Loki offered a deal with Laufey: he would let him murder Odin so that Loki would claim the throne for himself. To that end, Laufey sends over Grundroth and another Giant to guard the Bifrost Bridge and an incapacitated Heimdall (who was frozen by Loki). However, Heimdall was eventually able to break free of his icy prison and kill Grundroth by decapitating him by the neck while stabbing the other Giant in the chest.