The Swallow had come to a place where the river split into two.

"Which way now?" Sweetie Belle wondered.

Apple Bloom looked at Burble. "Yore a waterbeast, right? You ever been here before?"

Burble shook his head. "I've never been to this spot in my life. I couldn't be telling you which direction to take."

Twilight said, "Rainbow Dash was the only one who knew the exact location of the island."

Just then Scootaloo pointed with her hoof. "Look! There she is!"

Sure enough, Rainbow Dash was flying toward them. "Rainbow!" exclaimed Twilight. "We thought you were dead!"

"I thought so myself for a few minutes," said Rainbow Dash.

"I say, how did you survive?" said Joshy.

Rainbow Dash waved a hoof dismissively. "That's a long story. I'll tell you later."

"You came back at just the right time," said Twilight. "Do you know which way we should go?"

Rainbow Dash nodded. "Yeah, go left."

So, they took the left fork, but they hadn't gone very far when they came to a waterfall! The water was running straight over a cliff and down into a cave.

"Rainbow Dash!" screamed Twilight. "Why did you tell us to go this way? You trying to get us all killed?"

Burble tried to turn the steering wheel, but it was too late. The boat was sucked over the edge of the waterfall and plunged straight down underground.

Noise like none had ever known echoed and reverberated around them; darkness was everywhere. The boat spun through the air, down, down, down. There was a sudden impact. The Swallow struck an underground ledge with a rending crunch.

Twilight struggled to her feet. "Everybeast okay?"

"My head's achin', but I'm alive," said Song.

It turned out they were all fine, but the boat was totaled. There was a huge dent in the bottom; it would never sail again.

They stepped off the wreck and took stock of their surroundings. The cave was lit by a bluish white phosphorescent fungus that grew on the walls, so they could see they had landed on a ledge. The river continued to flow by on their right, and there was another huge pool of water on the left. Stalactite and stalagmite formations decorated the far reaches of the pool's edge.

Everybeast was furious at Rainbow Dash for leading them the wrong way. "What did ya bring us this way fer?" demanded Apple Bloom. "If'n Ah didn't know better, Ah'd think yew wuz misleadin' us on purpose!"

Rainbow Dash shifted uncomfortably. "Um… my bad?"

"Wait a minute," said Twilight. She took a closer look at Rainbow Dash. The light in the cave was dim, but she could tell something was off about her. Then it hit her. "The colors!"

"What?" said Song.

"The colors in her hair. They're in the wrong order!"

"What do you mean?" said Ocellus.

"The colors on a rainbow are always in the same order: red, orange, yellow, green, blue, indigo, violet. It's the same with the colors in Rainbow Dash's hair. But now, two of them are out of order. She's got blue and green mixed up!"

"What does that mean?" Sweetie Belle asked.

Ocellus narrowed her eyes. "It means this isn't the real Rainbow Dash at all, but an imposter."

The fake Rainbow Dash cursed. "Gluck it. I knew I'd mess up somehow." She turned into Coccyx!

"You're the one who stabbed King Thorax in the back," Ocellus said.

"I wish I hadn't," said Coccyx.

"Really?"

"Yeah, I should have gotten him in the chest. Then I could say I stabbed Thorax in the thorax!"

"We've got a score to settle, Coccyx," said Ocellus.

Coccyx sneered. "Easy for you to say. You're backed up by all your friends."

"My friends will stay out of it," Ocellus said bravely. "This will be between just you and me. I challenge you to a changeling's duel!"

"What's a changeling's duel?" Song whispered to Twilight.

Twilight explained. "It's when two changelings change themselves into different things in an attempt to destroy one another."

"You don't have the guts to take me on," Coccyx said to Ocellus.

"Yes, I do," Ocellus replied stubbornly.

"Fine!" said Coccyx. "I'll kill you like I killed Thorax!"

"You can't kill me like you murdered him. I'm facin' you."

Coccyx grinned nastily. "If we're going to do this, we need to agree on some rules."

"Rules indeed!" Apple Bloom shouted. "Yew only wants rules so's yew kin break 'em!"

Coccyx ignored her and focused her attention on Ocellus. "Rule number one, no vegetable or mineral. Only animal."

"Fine," said Ocellus.

"Rule number two, no magical creatures like pink dragons. Rule three, no disappearing."

"Rule four," said Ocellus. "No cheating."

Coccyx glared. "And finally, your little friends can't help you out. This is just between you and me."

Ocellus glared right back. "Of course. That goes without saying."

"Ocellus, are you sure you're up for this?" said Twilight.

Ocellus glanced at the alicorn. "Sure, I'm sure. That bully doesn't have the guts to take anybeast on in a fair fight."

"When did you become so brave?" said Twilight.

Ocellus smiled. "I guess it rubbed off from hanging around you."

"Enough stalling!" shouted Coccyx. And she changed herself into an alligator and sprang at Ocellus.

Ocellus had just enough time to transform before the alligator grabbed her. She turned into a turtle and bit the alligator on the finger. Coccyx screamed in pain and dropped Ocellus to the ground.

Ocellus tried to crawl away, but being a turtle, she was slow. The alligator was coming for her again. "Change to something else, Ocellus!" Song shouted.

Just before the alligator reached her again, Ocellus turned into a hare, just like Joshy! "Good choice," Joshy commented. Ocellus shot out her fist and punched Coccyx right on the snout!

Then Coccyx turned into a fox and started chasing Ocellus around. Ocellus made a run for the wreckage of the Swallow. When she reached the boat, she went into the galley.

Coccyx followed, pushing the kitchen door open. She looked around but couldn't see Ocellus anywhere. "Ah, ah, ah, Ocellus," she taunted. "No disappearing, remember?"

But then she saw Ocellus. Ocellus had turned into a caterpillar and was crawling up the wall.

With a shriek of joy, Coccyx changed herself into a chicken and flew into the air, intending to peck the caterpillar to death. But just as she got close to Ocellus, Ocellus turned into a walrus and flopped on top of Coccyx, squashing her.

However, Coccyx was not beaten yet. She proceeded to turn herself into an elephant, smashing through the roof of the boat's cabin in the process.

Coccyx grabbed the walrus that was Ocellus with her trunk. But suddenly the walrus changed into a mouse. Elephants are afraid of mice, so Coccyx dropped Ocellus and tried to run away. But she got stuck in the cabin door.

Ocellus then changed herself into a goat and butted Coccyx from behind. Coccyx went flying out the door, off the boat, and into the pool of water, disappearing with a splash.

Ocellus turned back into herself and flew back to her friends. She thought she had won.

"Way to go, Ocellus!" cheered Scootaloo.

"Yes, you kicked Coccyx right in the coccyx!" said Joshy.

"Shame about the further damage to the boat, though," said Burble.

But it wasn't over yet. The pool began to bubble. Then Coccyx rose up from beneath the water. She had changed herself into a giant purple dragon.

"Hey, you said no dragons!" Ocellus accused.

Coccyx grinned wickedly. "Did I say no purple dragons? Hm?"

She flew at Ocellus and grabbed her in her enormous dragon paws!

Song tried to run at Coccyx, but Apple Bloom grabbed her by the tail, holding her back. "Slow down at the pasture, racehorse," Apple Bloom said. "We ain't supposed to interfere, remember?"

"She already broke the rules by changing into a dragon!" Song snarled, struggling in the filly's grip.

But just then Ocellus seemingly vanished! "Hey, you can't disappear!" Coccyx shouted.

"I haven't," came the voice of Ocellus. "I'm still here in your hand. I'm just very tiny."

"What are you?" Coccyx demanded.

"I'm a germ. A rare disease found only in changelings. I'm called tumorsyphilisitisosis. And you just caught me, Coccyx!"

"WHAT?" Coccyx shouted.

"First, you break out into spots," Ocellus continued. Coccyx stared in horror as spots began to appear all over her body. "Followed by hot and cold flashes." Coccyx's skin turned feverish red, then ice white. "Then, violent sneezing."

"ACHOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!" Coccyx sneezed. She sneezed so hard that she fell over backwards and impaled her body on one of the stalagmites that was growing out of the ground. She died.

Ocellus poofed back into her own form again. "Not bad, huh?"

"You were brilliant." Song beamed.

"I'm just glad it's over," Ocellus said. "Now let's try to find our way out of this cave."