Spike was lying in his bed admiring the shiny shell the ponies got from the Hippocampuses.
As you remember, the Hippocampus emperor Pounding Surf gifted this enchanted shell to the ponies for their help in rebuilding their undersea city of Aquariopolis after it had been demolished by Princess Luna during a fit of rage after the Hippocampus Mages cast a spell that wound up hurting Luna's little friend Pipsqueak.
According to Pounding Surf, all the ponies have to do is drop the shell in the ocean and the Hippocampuses will get the word that the ponies need their help.
"Hmm," Spike hemmed as a thought occurred to him. He got out of bed and ran downstairs to Twilight's study.
Twilight was acting as host to a gathering of the Element Keepers.
"Hey Twilight?" Spike called, "I need to ask you something about this shell."
"Oh. All right, Spike. What do you want to know?"
"Well, it's just that the heepa-- hoopie-- hummina-- Well, the Sea Ponies gave us this shell to call them if we need their help, right? So, how are they going to call us if they ever need our help?"
Twilight had no answer for that, other than a long pause followed by a "huhm."
"Maybe we should go ask them," Fluttershy suggested.
"Well, I hate to use this shell for something that's not an emergency. But we really should have a way of knowing if the Hippocampuses need us. Okay. We'll go to the ocean tonight."
*** TONIGHT ***
The Six arrive at the ocean's edge.
"Okay. Ready?" Twilight asked, then dropped the shell into the ocean.
Moments later, Wavedancer and two Hippocampus guards rose up from the waves and swam to the shoreline.
"Greetings to you from Aquariopolis, surface friends." Wavedancer called to them.
"Why have you summoned us?" one of the guards asked.
"Oh. Okay, it's like this:" Twilight began, "A while ago, Emperor Pounding Surf gave us this shell to call on you if we need your help.
"Well, just this morning, Spike realized that you have no way of contacting us if you need our help."
"Odd that you should think about this just now." Wavedancer answered, "For Aquariopolis has indeed fallen on dark times and we were left wondering that very thing.
"It's was great fortuitousness that brought you here as quickly as you did."
"Oh no!" Twilight gasped, "What happened?"
"Our sovereign Emperor, the Glorious Pounding Surf," the other guard explained, "has lost the Trident of Rule."
Pinkie reacted in horror, "Oh no! Not the Trident of Rule! ANYTHING BUT THE TRIDENT OF RULE!!!!" Then she calmed and added, "What's the Trident of Rule?"*
Down in a secret cave with underwater access to the open ocean, the Six and Spike met with Brain Coral, Pounding Surf's majordomo (It's a word! Look it up!).
"The Trident of Rule is exactly what it sounds like." Brain Coral explained, "It is the symbol of Emperor Pounding Surf's sovereignty over the oceans.
"But three nights ago, an undersea quake ruptured the seal on the prison where the Tyrant Squirk was held."
"'Squirk'???" Pinkie shrieked, then fell to the cave floor in a fit of giggling.
"Do NOT make light of him!!!" Brain Coral warned.
Pinkie shut up, right there.
"The Tyrant Squirk has ears everywhere, and he HATES having his name being made a mockery of!"
Pinkie, cowed, got to her hooves and said, "oh... okay....."
Not that Pinkie was afraid of Squirk, but being told that making fun of his name made him feel bad... well, Pinkie doesn't want to (knowingly) make anyone feel bad.
"The Trident must be extremely powerful," Twilight reacted in marvel, "What kind of magic does it have?"
Brain Coral smiled sadly and shook his head, "None at all."
"Huh?" Twilight responded.
"But Tyrant Squirk believes that it does! And for this, our Emperor has been turned over to Squirk's chief interrogator, Krank, to be tortured into revealing its non-existent magical powers!"
Ponies and dragon gasped in outrage!
"This is why our city mourns! Not for the loss of the Trident, but for the abduction and for the suffering inflicted upon our Emperor!"