Demonic dive

Wandering the halls of a small fortress in Hell was a low-level demon lord, known as Relgudav. His twin tails flicked about in boredom as he looked for something to do, checking in on his servants along the way. He didn't have all that many, only a couple dozen, though there was still quite the variety; some had extra arms, some had wings (Which Relgudav had bound, as he lacked wings himself and wasn't about to be shown up by his own servants), one of them was even partly aquatic, and while it was a pain to get the water that one needed, they were helpful enough to be worth the investment. One in particular, however, was not. There was a crash from a nearby room, and Relgudav facepalmed. "Ugh, fuckin' Dran..." He went in to check on them, finding that they'd knocked over a metal statue that had dented to floor with its weight. "S-sorry, sir!", Dran stammered as he struggled to lift the statue.
Dran was an oddly normal looking demon among Relgudav's servants. Near-black skin, 2 arms, 2 legs, a tail that ended in the common "spade" shape, and a pair of horns. The only thing that really stood out about him was his trio of eyes, which was a somewhat rare thing.
Relgudav, nearly 4 times Dran's height, effortlessly picked up the statue and put it back in place, growling as he saw that it had been scratched by the fall. He was sick of Dran, who had been recommended to him by someone he'd thought was a friend, and been a nuisance ever since, making a major mistake like this almost every day. While Relgudav wanted to get rid of him, it wasn't as simple as firing or killing him; doing either would make Relgudav appear to the other demon lords as an ineffective master, and sabotage his chances of moving up in the hierarchy. No, if they wanted to get rid of Dran, they'd have to trick someone else into taking him, or hope that one of his mistakes ended up killing him. Although, there was one other way...
"...Dran." Relgudav looked over at him, and he nervously stepped back.
"S-s-sir?"
"Day after day, you infuriate me with your failures, and I tire of them. Redeem yourself to me, or your next failure will mark your end." Relgudav wouldn't be killing him, of course, but they wouldn't need to with what they had in mind.
"W-what, um... Would you have me do?" Dran fidgeted with his hands as he looked up at his master, worrying about what they might say.
"Go to Uggard Lake, and retrieve the reliquary from the cave inside. Don't come back until you have it."
"Y- Wait, you mean..." Dran had heard of Uggard Lake. It was full of lava instead of water, and the reliquary Relgudav mentioned was rumored to hold a portion of the wealth of the late demon king Asmodeus, who'd been slain in an attack on Heaven millenia ago. Said rumor, however, also included that the reliquary was guarded by a powerful monster, and none of those that had been sent to claim it had returned. This was almost certainly a death sentence, and Dran knew it, but it was either go there and give it a shot or be executed by his master here and now for disobeying.
"...Yes sir. I'll try." With that, Dran left the room, and Relgudav grinned. Finally, that worthless servant was gone, and he wondered what would happen to them first; would they drown in the lava, or be torn apart by the monster?

After a few hours of travel, Dran came to the moment he'd been dreading; his arrival at Uggard Lake. There was no one else around, which he'd expected, and he nervously waded into the lava. Thankfully, he was quite a bit more resistant to heat than most demons, and even lava only felt the way a hot tub would to a human! Soon, he'd start to swim, occasionally diving below the surface for a while to look for the cave. It took longer than expected, but he found it, as much as he didn't want to. With another trip to the surface and some deep breathing, Dran dove down again and swam into the cave.
The glow of the lava allowed him to see, and already there were occasional bones on the cave floor. Trying to just ignore them, the demon swam on, some bubbles leaking from his nostrils. He was good at holding his breath, sure, but unless there was air to be found in this cave, there were going to be issues in a few minutes. Nothing really happened for a while, though Dran sometimes heard (or thought he heard) distant sounds that could've been the monster.
Fear steadily crept into his mind, and so did a need to breathe. After about 5 minutes of holding his breath, Dran was starting to really struggle, pausing and rebreathing in his cheeks as he eyed up a fork in the path, one route going up and the other down. He couldn't just float here and waste time; he picked the downward path and hoped for the best. Another 30 seconds went by, but as he started to gain a purple blush, he came across an air pocket above him! The breathless demon quickly swam up and into it, gasping loudly before trying to quiet himself, worried he'd attract the monster's attention. Panting, he caught his breath, then held it again and dove once more, continuing his search.
The next little while was uneventful again, though with the bones lying below Dran becoming more common. Another 5 minutes in, starting to strain again, he entered an exceptionally large chamber, with many different tunnels all around. As he considered which one to go through, he heard a sort of deep groaning sound, seeing a light start to come from one of the tunnels. The monster was coming! Terrified, Dran glanced around the floor of the chamber, finding a boulder big enough for him to hide behind, which he did, and he plugged his nostrils to keep his bubbles from giving him away. Then, he just... Waited.
Dran's heart was pounding, and he did he best to keep quiet even as he squirmed from his need to breathe. He could tell the monster was taking its time going through this room; the glow of its body was barely visible to him, and was barely moving. Figuring it couldn't only be looking in his direction, he risked peeking at it. The monster resembled a bright red and orange shark with several long tendrils where each fin would be, was triple his height in length, and had seemingly a dozen eyes that were looking right at him! Dran put his other hand over his mouth to keep from screaming, and ducked back behind the boulder, for what good it would do.
A deep, ragged voice rung out. "Demon comes out?" Apparently the monster was intelligent and could talk, but was likely just trying to make an easy meal out of the panicked demon. He'd need to do something soon, his lungs were burning and he had a blue blush, tightly shutting his eyes and no longer able to silence his whines. Then, something brushed against him, and he yelped out bubbles. A few of the monster's tendrils were laying next to him... "Hold", the monster said, now floating above Dran. He looked up at it, and darted away, bubbles now streaming from his mouth and past his fingers. The monster followed him, staying above and keeping its tendrils near him. "Demon want breathe? Hold." The demon looked up at the monster's face, their multitude of eyes staring at him expectantly. He had to try.
Uncovering his mouth while still plugging his nostrils, Dran grabbed one of the tendrils and was pulled toward the rest of that group with an oddly gentle force. They wrapped around him, and one pressed against his mouth and nostrils to block them, pushing his other hand aside. Then, with him secure and confused, the monster charged toward a tunnel with incredible speed, so fast that the lava rushing across his eyes forced him to close them. He still desperately needed to breathe, and his body was trying to do so, but the tendril on his face kept him from sucking anything in. Still, he shuddered, starting to feel dizzy as his chest heaved back and forth, the tip of his tail starting to tingle with numbness...
"GHUUUUHHH!" The tendril lifted from Dran's face and he finally gasped as he was lifted into an air pocket! Panting heavily and coughing a few times, he was set down onto the floor that this air pocket apparently had. He took some time to recover, and the monster spoke again. "Sorry to scare. You... Different. Like master."
Having pretty much returned to breathing normally, Dran responded. "Um... I-it's okay... Who's your master?"
"As-moe...Day-us. Say watch treasure, me know when stop."
"A-asmo...! You knew him?"
"Made me. What 'knew' mean?"
"...He died, ages ago."
The monster, despite its face not being very expressive, was visibly saddened, its tendrils drooping. "...Master said might not come back... But, he really gone..."
The two were both silent for a moment, before Dran tried to change the subject. "I'm Dran, by the way. What's your name?"
"No have real name. Master call me guardian."
"Would you like a name?"
The monster perked up again. "Like name. You give?"
"Yep! How about, um... Akron?"
With some happy-looking waves of tendrils, it was clear that the monster liked their name before they even said so. "Good name! Me Akron, thank Dran!"
"Alright, cool! Now, could you maybe take me to the treasure?"
Akron shook their head. "Too far, Dran drown. Stay, Akron bring." Without waiting for a response, they dove back into the lava, leaving Dran alone. For about 10 minutes, he sat and waited, his breathing very slowly getting heavier as the air got stale. Finally, Akron returned, a large golden box held in two sets of tendrils. "This treasure. Dran get on back, take outside." The tendrils on their back loosened, and Dran climbed on, allowing them to wrap around him and lay him on Akron's back. "Hold breath."
With one more deep breath, the two submerged and shot toward the cave's exit, and in just a couple minutes, they were outside. Akron went near to the shore and released Dran, who hopped off into the lava that still came up to his neck and grabbed the box from Akron, struggling a bit with its weight. "What do with treasure?"
"My master sent me here to get it. I think they might kill me if I come back without it."
"Your master mean, should get on own. Dran should keep."
"...You know what? Yeah, screw him. People have been trying to get this for ages, I'm not gonna give someone else a chance to take credit."
Setting the box down on the shore, Dran opened it to find it was packed with gems! With the demon hierarchy being somewhat based on wealth, he'd become a demon lord for sure, and be well on his way to surpassing his ex-master.
"Is good?", Akron called out, unable to see the box's contents from where they were, and not even able to get closer because of how shallow the lava became.
"It's great! I'm gonna be famous!"
"Dran still visit Akron when famous?"
"Of course! I'm not about to forget the guy that made this happen in the first place."
Dran closed the box again, and lifted it back up. "See you later, Akron!"
"See later, Dran!" Akron wasn't entirely clear on what that meant, but quickly figured out it was essentially "goodbye" as Dran started walking away, carrying the box with him. After briefly watching him, Akron swam out to a deeper section of the lava lake and dove back under, happy to have made a friend and finally finished their mission.