Thor, Rocket, and Groot arrive at Nidavellir to find the great forge silent, the dwarves dead, and Eitri in mourning. But since it was Thanos who murdered his people, Eitri is willing to help, so long as those who have come to see him grant him the means by which he can create a Thanos-killing kind of weapon.
When Loki arrives at Nidavellir, he is tired, hungry, and Sleipnir is miserable from not having space to stretch. The great rings of the forge are dark and this disturbs him. He lands and lets Sleipnir out of the ship, his magic steadying the great beast as he takes the first unsteady steps he's really taken in days. He was able to stand in the ship, though barely, and could hardly move. Nidavellir is spacious, even though it is dark.
They follow the sounds of the others and watch in awe as the bright heart of the star springs to life, Loki whispering in Sleipnir's ear, "So few living have been blessed to see Nidavellir in all it's glory, my child. I did, once. And I paid for my insolence, arrogance, and trickery when I did not give Eitri the respect he and his people deserve. Even though it is much-changed, what you see now is still a wonder and you will likely never see anything like it again, no matter how many incredible place we visit in our long lives." And then the light dies and the forge stills. Loki and Sleipnir continue following the sounds, looking for Eitri.
When they do find him, they hear him shout that the iris is damaged, Thor replying, "How long will it take to heat?"
"A few minutes, maybe more. Why?"
"I'm gonna hold it open," Thor says.
"That's suicide," Eitri replies.
And Thor answers, "So is facing Thanos without that axe." Loki runs forward, Sleipnir on his heels, as he watches his brother hurry toward the iris.
"Thor, no!" Loki shouts, "Wait!"
Eitri turns to see Loki and Sleipnir, but puts his arm out to stop them from reaching Thor, "It may be a terrible plan, but it's the only one we have!"
"But I could use magic-"
"No, you can't, boy!" Eitri yells, cutting him off, "None of that works the same way here and if you try, it could contaminate the magic this all draws from and weaken it! For once in your life, think something through, Loki."
Loki freezes, stunned by Eitri's thundering voice, "I...I just want to help."
"Given what happened last time you were here, I find that hard to believe," Eitri says, "So stay back. This can't go wrong for any reason. No biting flies this time."
Loki shrinks against Sleipnir's side, remembering the tricks he played on the dwarves so very long ago. His fingers drift to his lips as he feels the memories of steel puncturing them, stitches binding them closed. He weaves his fingers in Sleipnir's mane, comforted by the warmth as Sleipnir nuzzles his cheek.
He hears Thor's voice, "Allfathers, give me strength."
Loki buries his face against Sleipnir, whispering a prayer to no gods in particular, "Please, don't let him die. I just got him back... Let this work. It has to work. And he has to live."
"You understand, boy? You're about to take the full force of a star. It'll kill you." Eitri replies to Thor.
"Only if I die," Loki hears Thor reply.
Loki shakes his head and quietly laughs as tears creep into the corners of his eyes, "You ridiculous, noble, fool..."
Eitri shakes his head, "Yes. That's what killing you means..."
Loki doesn't want to watch his brother burn, but he feels as though he should also honour this sacrifice, if he does. He wrestles with the decision a few moments before he hears Eitri call out.
"Hold it! Hold it, Thor!"
Loki looks up, first to Eitri, watching the metal melt, pouring into the massive mould, and then to Thor, who strains against the heat of the star. When his strength fails, he is swept inward to the forge, and Loki rushes to him, Sleipnir on his heels. He gathers Thor in his arms. When Rocket lands his pod and runs to them, he hears Loki whispering something to Thor in a language he can't understand.
"Thor!" Rocket calls, "Say something. Come on. Thor, you okay?" He yells to Eitri, "I think he's dying!"
"He needs the axe! Where's the handle? Tree, help me find the handle!" Eitri bellows.
While Groot grows himself into Stormbreaker and breaks free from it, Loki can only focus on ancient prayers his mother taught him, incantations to focus one's mind, call on any deities that might be listening, and to prepare oneself for difficult decisions.
Rocket rests his hand on Loki's arm, "Hey...he's gonna be alright, right? He has to be. He's the hero...not guys like me." Thor's fingers twitch and small sparks of lightning dance at their tips. Rocket's eyes grow wide, "Loki, I think you might want to put him down!"
Loki notices the movement, feels the air tremble as Stormbreaker comes to its new master, and has just enough time to shimmer into a more appropriate set of clothing for fighting before the Bifrost envelops all four of them and he knows they are being transported to wherever Thor has asked for them to go.