Before leaving, Nathan summoned Nidhogg, the ace he had hidden this far. He showed the chain, asking, "Can you destroy this? Maybe with your fire or something. But make it in a way that won't hurt my hand."
Nidhogg nodded and bit the shackle, controlling his fire at the back of his stomach so that he only melted the path between his jaws.
The shackle reddened for a second and looked like the heat was transferred to the entire shackle, but Nidhogg immediately broke it with his jaws and threw it away.
He then proceeded with the rest of the shackles.
On the other hand, Sera was completely stunned by Nidhogg's appearance. A wolf and a jackal were indeed surprising for a summoner, but a dragon? It was basically the pinnacle of beasts.
By the time she got over her shock, all the shackles had been crushed by Nidhogg. Nathan pointed at her, saying, "Free her too."