A/N: I wrote this right after I saw Infinity War, but I like to give people a chance to see the movie before I post.
Elsewhere
Several of the heroes were confused when they began materializing in the otherworld. Sam Wilson had been so dazed from a crash landing he hadn't even realized he was dissolving until he wavered to his feet on a featureless plain with flat gray ground and faintly orange sky. "What the hell?" he said, as other familiar faces began to appear around him.
T'Challa pulled off his mask. "I have no idea," said the king, who hadn't seen anyone else turn to ash, before he fell headfirst into nothingness.
"We dissolved," said Bucky Barnes flatly. He had seen his feet and body go before he was suddenly elsewhere, leaving him, for the second time in his life, with an image of Steve Rogers' horrified face.
"Thanos won," Wanda said wearily. T'Challa caught her hand to pull her to her feet. "I destroyed the mind stone, but Thanos turned back time and pulled it out of Vision's head, so he had a full set. Then Thor axed Thanos, but didn't kill him instantly, so Thanos had time to snap his fingers and kill half the people in the universe just like that."
This was half of the Avengers that had been on the field in Wakanda, they all realized.
"Kill? This doesn't look much like the heaven Reverend Mitchell preached about," Sam said.
"Purgatory," suggested Bucky who had been an altar boy, long ago.
It didn't look like the plains of grass T'Challa's father had taught him to expect, but the king focused on something else Wanda had said.
"Half the people in the universe? This place could get very crowded," he said. But no one else appeared, not even any Wakandan warriors, just the Avengers and a frightened tree-teen huddled with his arms wrapped around his knees.
"Not seeing anybody," Sam said.
"I'm sensing someone, a few someones, that way." Wanda pointed.
"Steve?" Sam asked. "Natasha? Stark?"
"No, no one I know. And yet, one seems familiar," she mused.
"Let's go see," T'Challa suggested.
Bucky held out his flesh hand to the tree person. "Come on, twig," he said kindly. "We should stick together."
"I am Groot," Groot said, as he took the hand.
"Pleased to meet you. I am Bucky Barnes," Bucky answered.
The group followed Wanda's lead and, sooner than should have been possible, they came within sight of another group that looked just as rattled as the Avengers. Groot tore his grip from Bucky's and ran to wrap his arms around Peter Quill.
"Hey, it's OK," Quill soothed. "Well, as OK as being dead can be, I guess. Starlord tallied his team and sighed. "Poor Rocket. He's all alone now."
"Is that the raccoon who wanted my arm?" Bucky asked, gesturing with his cybernetic arm. "My friends will take care of him."
"Who would that be?" Quill asked, though he had a guess.
"They're, uh, the Avengers," Peter Parker offered, coming from behind Drax. He waved sheepishly. "Hi, guys."
"Hey, it's Spider-kid, right?" Sam teased.
"Spider-man," Peter corrected huffily.
"Stark drag you into danger again?" Bucky growled.
"No! He tried to send me home," Peter defended his mentor. "But I wouldn't, I couldn't leave when he needed me. Anyway, I guess I'd've dissolved back in Queens just as well as on Titan."
"Probably true," T'Challa agreed.
"So, these are more Avengers, like you?" Quill asked.
"Like you?" Sam raised his eyebrows at the youngster.
"Mr. Stark said I was an Avenger," Peter defended himself.
"OK." Sam nodded agreement. "He was the official leader at the time."
"We are approximately half of the Avengers," Wanda told Quill, emphasizing the "half."
"Yeah," Quill said bitterly. "Thanos boasted about removing half of the people in the universe to save us from ourselves, but I think he biased the spell or whatever it is. This is about 80 percent of the Guardians of the Galaxy, and he already killed one of ours to get the soul stone."
"I'm sorry, man," Sam said sympathetically.
"We failed," T'Challa said sadly. "We tried to protect the mind stone, but we failed."
Wanda told the Guardians everything that she had witnessed.
When they heard that the time stone had given the mind stone to Thanos, the two Peters winced. "It's not on you," Quill said heavily. "We failed to protect the time stone. And, apparently, we led Thanos right to the soul stone when we let him get hold of Gamora."
"She was the only one who knew where it was," Mantis explained.
"Remember, Dr. Strange said this is the way it had to be," Peter Parker reminded the others.
"Dr. Strange?" T'Challa asked.
"He's the wizard who had the time stone," Peter explained. "He checked a million possible futures and we only win in one."
"And that's this one?" Sam said incredulously.
"We hope so?" Peter said. It was more a question than a confident statement.
"Where is this wizard?" T'Challa asked.
Seated in the lotus position, Dr. Strange rose into view and hovered above the gathering of Guardians and Avengers. In his driest chairman of the board voice, the Sorcerer Supreme said, "I suppose you're wondering why I called you all here today."
A/N: Because this is the only way they can win.