I think I'm understanding something about Akatsuki no Yona after reading the manga and watching the anime lots and lots of time. I think I know what or who the Sword and Shield might be from the prophecy told by Ik-Soo.Let's analyze the prophecy, shall we?

"Darkness has fallen upon the land.

The blood of the dragons will revive once again, 

and the ancient pact will be kept.

When the four dragons are gathered, 

The sword and the shield which will protect the king shall awaken,

And the Red Dragon shall restore the dawn at last."

Just by looking at this, I'd better tell you... MAJOR SPOILERS AHEAD.

You have been warned.

 OK, now let's think of the shield mentioned in the prophecy. It's Zeno. Why? Because when Zeno's skin turns into dragon scales after getting through several injuries, his body regenerates to steel... STEEL... What do you think  a shield is made from. Oh yeah, from steel. OK, let's look at how everytime Yona or the other dragons are in danger, it's Zeno who SHIELDS them. Of course dragons protect each other but when no other dragons could protect Yona, it's Zeno who does the job. He regenerates himself for that very purpose... To protect Yona from harm just like the other dragons do but he regenerates into steel. Other dragons also plays the part of the shield but it's Zeno being a huge role being a shield. There's also the ending song. singing about all the dragons. It goes something like this : 


The ferocious arm (meaning Ki-ja)

The eyes that see a thousand league (meaning Shin-Ah)

The legs that could run through the skies (meaning Jae-Ha)

And the ETERNAL SHIELD (meaning Zeno) 

Check it out.

 Which means, the sword may be someone associated to the dragons. As for the sword, I have two theories about it...

THEORY 1: The sword maybe Soo Won. There must be a reason he killed Yona's dad but in my opinion, I think Soo Won was told to kill by the King... Maybe...

THEORY 2: The sword is currently with Soo Won. If you watched the anime, at the end of the 1st opening song, we can clearly see Soo Won holding on to Yona and in his other hand, he's holding onto a sword... As if giving it to her. And, there's this one episode... Episode 23, when Soo Won hides Yona from his men. She saw a sword but unable to take it and kill him because it's found she still has some affection for him. But is it just affections... or was the sword preventing it? Which leads to my next theory about another part of the prophecy. I'll get into that real soon. But for now, I hope you guys might be able to express your opinions and I also hope this article might open up a few other doors. Personally, I think Akatsuki no Yona is the most mysterious manga/anime I have ever read. Thanks for taking the time to read this. I have way too much free time on my hands.