After failing to convince Uglúk to lead the expedition east to safety in Mordor, he leaves and returns with 20-40 peers from Mordor, claiming a brotherly desire to help their fellow Orcs. However, Grishnákh's actual plans for the two captives, Merry and Pippin, were in conflict with Uglúk's orders to deliver them to Saruman unharmed. He was an Orc of considerable rank, as he had been to Barad-dûr itself and received specific orders from the Nazgûl. Grishnákh was also present at the torture and interrogation of Gollum, so he not only knew about the Ring, but suspected the Hobbits might have it after hearing them imitate Gollum's throat noise. Afterwards, he tried to smuggle the Hobbits away from the Uruk-hai and into Fangorn Forest, where he planned to kill them and take the Ring for himself, but he is killed by a spear from a Rohan rider.
In Peter Jackson's movies, Grishnákh (played by Stephen Ure) is shown to be the captain of a band of Orc scouts, possibly from Isengard as they wear the attire of Saruman's warg-riders seen later in the film. He and his fellow Orcs meet Uglúk's group in the western Emyn Muil rather than at Amon Hen as in the book. That night he fights with Uglúk over the Hobbits, as he and his party want to eat the Hobbits instead of delivering them to Saruman. He is speared as in the book, but survives to chase Merry and Pippin into Fangorn Forest, where he is killed by Treebeard, who steps on him before he can kill Merry. His name is never actually spoken in the movie, and it is uncertain if he knows whether the captive Hobbits have the Ring. In the film it is not clear whether Grishnákh and his company have been sent by Sauron from Mordor or are workers from the mines at Isengard.