Frank and Rainbow zoomed across the ocean. The hippocampus was large, fast, and knew exactly where he was going. He cut a clear path through the salty water, faster than a speed boat. Frank relaxed on his smooth back and let the sea spray his face. California soon vanished behind him.

After a while, Frank lost track of how long they'd been travelling. It was an hour or two at least. All that was visible was ocean in every direction. If he squinted hard enough, he could see a very faint coastline in the distance. He didn't know where they were though. Rainbow began to slow down, and soon came to a stop.

'Is this it? Are we there?'

Rainbow neighed enthusiastically. Frank took that as a yes.

'Okay. Here I go.'

Rainbow kicked his tail up into the air and took a nose-dive. As he was plunged underwater, Frank began his first transformation – an octopus – as he got his first glimpse of the underwater city of Atlantis.

It was worth going on this crazy quest just to see Poseidon's palace. The sight took his breath away. The whole place was easily as big as New Rome, and maybe Camp Jupiter too. If he were Percy, he'd be tempted just to come and live here. It would have been the most wonderful place he'd ever seen – if not for the ginormous shrimp-cockroach hybrid that was tearing it apart.

The sight of the Skolopendra almost made him want to turn on his heels (tentacles?) and swim in the opposite direction. It was exactly as he remembered – face resembling the ugliest catfish in the world, glassy dead eyes, and the worst case of nose hair he'd ever seen. Annabeth had told him it was a child of Phorcys and Keto. How those two had managed to spawn this thing, Frank had no idea. He swam closer to get a better look, one of his eight legs curled firmly around the vial of Greek Fire.

It was currently destroying one of the gardens, sending pieces of coral and sea anemone flying everywhere and making a tremendous noise. There was a path of destruction which showed where it had been. Buildings that had once been smooth, glistening abalone shell now had chunks missing, smashed by a flat crayfish tail. They were now shining half-heartedly, pieces strewn across the ground.

They were obviously trying to fend off the beast…and not having any luck. Sea creatures and merfolk were being whacked by its tentacles and sent careening off into walls or the sandy ocean floor. Tridents, spears, and other weapons bounced harmlessly off its pink chitinous shell. This only seemed to irritate it more. The bravest fighters were managing to slice off a tentacle or two, but more quickly sprouted up in their place. Realising this, the mer-warriors quickly gave up.

His best bet was to start by distracting the monster and stopping it from wreaking havoc on the palace and all those merfolk. Since he was using Greek Fire, he'd have to be careful not to cause any damage to the place himself. Poseidon probably wouldn't appreciate that. That meant he'd have to lure it away – by turning into something big enough to challenge it. Maybe a blue whale or an orca? Not intimidating enough. He needed to be something that could fight it. Something…not too different from it.

He'd also need to let the small army know that he wasn't a threat, so that they wouldn't start aiming their tridents and spears at him. It would, therefore, be a good idea to get close enough for them to see him transform. Hopefully they'd know about shapeshifters.

He propelled himself through the water until he was right in front of it. Nobody paid him any attention – they were all too busy attacking or being attacked. His octopus form was very small, and he felt like a dot next to it, a tiny drop in the ocean. But not for long. He thought back to the encounter with Phorcys and Keto and pictured one of the creatures he'd seen in the tanks – a squid the size of a lorry with a beak like a bolt cutter. His body grew until he was staring the monster in the face. Then he extended a tentacle and whacked it in the eye.

This did the trick. It turned on him in outrage, as if unable to believe that anything would dare challenge it. Frank edged away, like Come and get me! He heard a few shouts of surprise from around them and guessed the merpeople were very confused. But they weren't attacking him, which was a good sign.

A nose hair nearly whacked him back in retribution. He squirted thick black ink in its face, making it bellow in rage. While it was confused and dazed, he kept luring it further and further away from the palace. He poked and bit at it with his beak. The Skolopendra was now very angry, and it was bigger than him. The giant squid wasn't quite giant enough. But Frank wasn't a child of Mars for nothing. He used his own tentacles to grapple and wrestle with it.

The army of merfolk had frozen and were watching in awe at the fight going on in front of them. Most of them had lowered their weapons, realising they weren't needed anymore. Others were staring apprehensively at Frank, wondering who or what he was.

The fight was going well so far, now he needed to think about the second and most important part of his plan – getting it to swallow the Greek Fire. And for that, he'd need to get it to open its mouth.

He recounted the voice of Bythos.

'The koi fish is good, but try for a Portuguese man-of-war. Remember what I showed you. It's all in the breathing.'

At the time, he'd had no idea what that even was, but he'd looked it up, and now, it was time to try turning into one. It was just what he needed. He took a deep breath in, and then out, and then felt himself shrinking. His head became large, transparent, and bloated like a balloon. The rest of his body transformed into long, sweeping blue tentacles that curled like springs, each laced with venom, cool and deadly. The vial was still firmly clutched in one of them.

Shrimpzilla paused in the water, wondering where its opponent had gone, tentacles still undulating hypnotically. Then it noticed the blue bottle jellyfish hovering next to it and lashed out, seemingly unaware of the danger. But Frank dived out of the way – the moment wasn't right. He had to time this perfectly.

He swam around its head a few times, making it spin around like a dog chasing its own tail. When it was slowing down, evidently dizzy and tired, Frank positioned himself right in front of its face, as close to its mouth as he could safely get. Even the tentacles weren't thrashing so much now – just what he needed. He extended one of his own, grabbed the nearest nose hair and felt the venom leave his body, giving it a sharp sting.

The tentacle shuddered violently, its slimy pink face contorted, and its gaping toothless maw opened wide as the beast screeched in pain. Seizing what was possibly his only chance, Frank let go of the vial, turned into a shark, and used his strong tail to whack it down the creature's throat.

Having been underwater the first time this had happened, he didn't know what to expect. Whatever he'd been imagining didn't come close to the real thing. The screech of pain turned into a horrible gagging noise. Then there came a muffled explosion from inside, and Frank could have sworn that its expression said, Not this again! The nearby palace buildings were bathed in green light as its body lit up like a lantern. With one last convulsion, it regarded him with disgust, turned, and began to make its retreat. It swam until it was just a fleck in the distance.

The army were all gawping open-mouthed. Some began to clap hesitantly, while others broke into conversations about whether that had really happened or not. Many of them picked up their weapons, which they had dropped in surprise, and swam off to tell their friends what they'd just seen.

Shrimpzilla wouldn't be making a comeback anytime soon – but that didn't solve the problem of Phorcys and Keto. Surely they'd send something else to take its place.

After most of the audience had disbanded, Frank saw someone approaching him. As they got closer, it became clear that it was a merman with two tails, green skin, and sleek black hair in a ponytail. Triton, he realised.

The merman stopped in front of him.

'Not bad, Frank Zhang. Not bad at all. Father is impressed.'

His voice sounded like a kindergarten teacher praising a five-year-old for writing his own name. Nonetheless, Frank swelled with pride. He opened his mouth to respond, but bubbles just floated out. Instead, he projected his thoughts to the minor god floating before him.

He is? Can I meet him?

'Oh, yes. But not yet. We have one more small task…if you are up to it.' He began to see what Percy had been talking about when describing his half-brother. He did seem a little big-headed. But who cared? After defeating Shrimpzilla, surely he could do anything.

Of course. What is it?

'Phorcys and Keto are nearby, we can sense it. But they have kept themselves very well hidden, and Father is much too busy to go looking for them himself.' His smug expression and tone seemed to say And so am I. 'Take a look around and see if you can find where they are hiding. I will give you some guidance for where to go. Then report back. And don't take all day.'

Frank was starting to dislike this merman. He would do it. He would prove himself. Again.

I won't let you down.