Meanwhile.

He didn't sneak away from the mission because he is too lazy to go, although he is quite lazy overall, but it was because he had something buzzing in his mind for a while: one lake, three unaware time-travelers from 3 different ages that all found themselves in the future after entering in contact with that water.
He decides not to tell anything to the others, since he can't be sure if he is right and he is not the kind of person who can accept making a mistake in front of people he knows. There is only one person he feels forced to bring with him and surely not because he has a special relationship with him: Amiir.

Some time before the others left for their own tasks, he and Amiir leave for their own. BangSul teleport them in South Korea, right outside the city of Seoul, in an empty meadow siding the highway trait that lead to the capital. There is nothing but grass there, not even cars driving by.
BangSul looks around and walks in the high grass till reaching a spot. "Once there was a big lake here. And there was also an inhabited village, which means you can open a portal randomly, we need to move somewhere shrouded." He starts to walk towards the mountainside, followed by Amiir who doesn't say a single word.
When in a place that seems optimal, the Hwangsang opens the portal indeed, leading at about 350 years in the past. He didn't choose the time randomly, he wanted the period to be close to when he 'died', but not enough to risk someone to recognize him. Time traveling to his original times is always a big temptation, to go seek revenge for everything he went through, and he knows that's something he should absolutely not do.

As expected, the lake he remembered is still there and so is the village, although he looks less lively than it was when he was still alive. He'd love to go get the remaining people devoured by tigers, but he has to restrain himself.
They start to walk closer to the lake, BangSul keeping a very thoughtful attitude all the time. When close to the shore, he looks at Amiir. "Can you feel it?"
"Ye..." murmurs the other.
His eyes widen. "Then I was right, for real! The legends of my times stated that a 'star' fell from the skies into the lake. Now it all makes sense..."
"Why did you bring me here?" asks Amiir, seemingly worried.
BangSul takes a breath. "Because I don't think I'd manage to take it. The other time I was in contact with that water... I remember a green light coming from the bottom, then I simply woke up at 400 years in the future. If I'll try to get it, it will probably relocate me in time again, but you are the Avatar of Time, if you can't subdue it to your power, nobody can."
The southern sends a scared look at the deep and dark water of the lake. "I can't swim..."
"Shii!" he swears. But before he could think of some plan to overcome this huge problem, he notices a weird man approaching the lake, that definitely doesn't look like any normal citizen of Joseon. Obviously, if the gem of time was in the hands of the wizards, someone had to retrieve it from that lake at some point and that seems to be that exact time. Unless their coming triggered something.

"Amiir..." BangSul says while keeping his hands on the man. "All you have to do is to push youself towards the gem, grab it and teleport back to the right time, I know you can do it. Don't wait for me, just do as I told you." Said that, he takes his spear from his back - when it's sheated it is smaller and returns in normal size when taken - and approaches the man to face him.
Taking the weapon also makes the armor appear on the Avatar*
The opponent looks at him from head to toe. "Are you lost, traveler?" says the man, sounding rather amused and as if he clearly knows what is going on.
"Quit this nonsense. We both know what we are here for", he growls.
The other tilts his head. "Your soul is split. I don't manage to figure out what happened to it."
"It's not split, it's fused with two other souls."
"Hm. But I still can't let you have that treasure. I doubt you can understand what it is or maybe you can, which makes it even more critical for me to prevent you from having it." The man raises his hands, summoning one of the familiar 'shields' wizards use to fight.
BangSul shakes his head. "I won't let anyone stop me."
And they start to fight for quite a while.
The wizard is eventually overpowered by BangSul, who sends him back-on-the-ground and points his spear at his neck. "Don't force me to do it. I shouldn't kill you, but if I'm forced to choose between the potential destruction of the future of this world and the return of the only thing that matters to me, I won't hesitate."
"Hasn't your friend been underwater for a little too long?" asks the wizard, as if ignoring everything BangSul said.
"He probably left already. That was the plan."
"Without the Infinity Gem? Because I can assure you, it's still under that lake."
He glances at the surface, that is not even slightly moved and swears, moves the spear away and runs towards the lake, quickly summoning the Shark Guardian to swim fast.
He reaches Amiir within seconds and finds him on the brink of falling unconscious. He grabs him and pulls him towards the bottom, where a green glow is well evident even in the dark. He shakes him to force him to keep consciousness for those extra moment and eventually, the other grabs the gem and teleports them back into their time.

The two find themselves in the tall grass, panting and coughing.
BangSul takes some time to recover then kneels up, taking a look at Amiir, who is coughing water out. Except from that, he seems all right. "Do you have it?" he pants, almost scared to ask.
Amiir slowly opens his hand, from which the green glow immediately appears and the other sighs in relief. "Thank the Gods..." he mutters, then caresses Amiir's head. "Good job. Let's go back now." He stands up and grabs his arm to pull him up as well, then teleports them back to the Avengers' hq.
At about the same time as this happened, the gem disappeared from the Ancient, just as she was handing it to Hulk.

When they return, BangSul decides to not tell anything about it to the Avengers who remained behind and just wait for when the others will come back with the other gems. Someone has very little faith in people.