Astrid, Raiden and The Twins.

The twins listened to the Seer, not understanding a word she said. They looked to each other in confusion, but neither of them realized the other one was confused, they just got angry because they were looking at each other with a stupid face. They began to argue which led to fighting. Astrid listened to the Seer trying to make sense of what she was saying, at first it just sounded like gibberish from a crazy person but even though something was telling her to ignore her, her instincts told her there was more to her than meets the eye, and thanks to the twins, she was unable to focus probably with their constant arguing and fighting. She began to get angry and walked over to the twins. She grabbed both of their heads and slammed them together as hard as she could, she them amplified the sound of their heads colliding, this would deathen them temperately, and the sound waves would cause a minor brain damage causing loss of short term memory and at the moment of impact she froze both of their heads together in one big block of ice, Then holding onto the block of ice with her right hand, she span around and sweeped their legs from under them with her left leg while throwing them with her right hand. After the twins was out of Astrid's hair, she was finally able to calm down as she took a deep breath and just as she was about to start thinking about the riddle again, she noticed Raiden was thinking hard about something and decided to leave it to him, while this was happening, Raiden was listening carefully to the Seer and trying to dither the riddle. Once she was finished he went through the riddle verse by verse in his head. 

Our time has come which means it is time for the drum. Up in the skies, dragons cry and die. This is probably referring to the final battle we had we the dragon queen. 

All we can do, is try and try, but after attempt after attempt, trying to save the dragon, we are in vain. It will all end in pain. This may be referring to when we saved Night Shade, but if that is the case it means he is still in danger. 

Heed my word, heed my warning. Listen to my rhymes, watch out for those crimes. Dagur comes, Dagur goes, everything gone, just a pawn, nothing more. This seems to be implying that Dagur was not responsible for this attack but rather a pawn for somebody else meaning either he teamed up with someone who is manipulating him or somebody knew he was coming early and set this attack to frame him but considering Dagur hates working with anyone it is probably the ladder. 

During the brightest moons, the tunes shall sweep the land, rendering all into a deep sleep. During the hottest suns, the sons are summoned. This seems to be saying something is going to put everyone in this land to sleep, apparently some sort of magic flute, and during the hottest day of the year, someone's sons shall be summoned. 

 Be warned, be warned, fear the dragon for the wagon is a trick. Islands sink, trouble underground. What is below is a mighty foe. This verse is telling us to fear a specific dragon which is a little worrying since we already defeated the queen, so what could be stronger than that? Not sure what the wagon means or which islands are supposed to sink but considering we are supposed to be warned, it is a safe bet to assume it was berk. This seems to suggest that either there is now or will be a powerful enemy underground, probably whatever is going to try and sink us, we will have to check that out. 

One here, one there, all is fair, with a dare. A single glare, a frightful scare. A ghostly figure, girl in black, mark of a Dragon Slayer, the final attack. This seems to be referring to the Seer herself and somebody with a mark of a dragon slayer whatever that is. Now, either dragon slayer is supposed to be leading this final attack against a dragon, hopefully the one who is supposed to sink us or it could be a final attack of dragons trying to attack us.

Wicked Witch, banished long ago. With magic great and powerful, sent far below. Now back again, blind as a bat. Full of rage, full of hate, now set the bait. This again maybe to referring to the Seer, meaning she may have a alternate motive, and this riddle may be the bait. 

Brothers and sisters, good and evil. Ying or yang, one is missing. New foes, new faces. Dagur's land, is no more. Be warned be warned, for I speak the truth. Solve this riddle, listen to the rhymes, be warned, be warned. Not sure what she means by the brother and sister thing or rather I don't know who they are talking about. It's clear that they are exact opposite of each other and one of them is missing which could be bad depending which one is missing as in that type of situation the two opposite personalities balance each other out effectively keeping them in check. It is obvious Berk has new enemies who maybe the ones who are trying to set Dagur up. The part of Dagur's land being no more is pretty straight forward. Obviously something's happened there, most likely someone or something has attacked. He then walked up to Dagur and Rogue and took them to one side to discus his theory in private where the Seer cannot hear. Once alone he looked at them and said.

"This is just a basic theory, as I have just given this riddle a quick thought and this is literally off the top of my head."

Raiden then explained theory, 

Dagur 

After hearing this Dagur looked to the chief and said,

"Sorry, I have to check on my territory, we will have to sign the treaty later."

He then turned away and then headed to his ship, both containing and hiding his excitement thinking to himself that this couldn't of gone better, who cares if my island has been attacked, any damage can be repaired, and if it has been destroyed I can relocate to Berk when I conquer it, but it gave him a perfect accuse not to sign the treaty. When he got to his ship he began to prepare to set sail.