Aquarius Diadem Quest
It's a curious top headgear that grants +2 STR, FLEE +10, and increases wind resistance by 5%. Whenever the diadem is refined to +7 - +10 it also grants DEF + 1 and ATK +15!
- 150x Worn-out Prison Uniforms
- 50x Little Evil Horn
- 25x Turtle Shell
- 25x Dragon Skin
- 10x Dragon Tail
- 15x 1 Carat Diamond
- 3x Cracked Diamond
- 10x Steel
- 5x Witch Starsand
- 1x Circlet 
You will need to head to Prontera Castle and head upwards 2 screens and head 1 screen to the left. You should end up in front of the gentleman known as Recruiter Lucane (66, 115)! He will ask you if you are of a worthy type, ready to join the chivalry! Answer him as such and show him that you are!
After you have talked to Recruiter Lucane, he will direct you towards Pelleus (prt_castle 85, 52) to educate yourself on a bit of Rune-Midgard history. Otherwise known as your 'test' of knowledge! This will be simple enough, go back down a screen or two and on the west (left) you'll see Pelleus reading from his book. Go and say hello, after a simple short conversation he'll send you on your way to see Friar Brumear.
To meet Friar Brumear (prt_in 172, 66) you will need to head back into Prontera Center, from the center statue head north and to the slight east before you get to the outer ring of Prontera you'll find a library on the right. Go say hello to the dear Friar inside. This will be a fairly rapid and simple conversation, and after a few short words you'll find yourself going right along, ready to begin the next part of the qualification!
Now for the real challenge, it's time for you to meet Kay (prt_in 71, 96); your new pain in the butt instructor! He'll inform you that only the strong may proceed and give you a list of items to retrieve. He is located inside of the Prontera Knight's Academy to the north west corner of Prontera.
After you've collected him your 'proof of strength' it's time to be congratulated. Head to Prontera castle. On the right hand side (eastern) when you enter you'll come to a room with a man inside a cage with Lord Siegfried (prt_castle 158, 29) waiting to congratulate you...
...as well as tell you the king requests your audience...
...or discover that you are the first recruit to ever pass all the tests! King Heinrich V (prt_castle 85, 159) is located in the final map of the castle before entering the Guild Castle map. He will tell you how very busy his schedule is standing around there; but award you the Aquarius Diadem (as well as a nice chunk of experience) as his thanks and congratulations for being the first person to pass all the Chivalry tests!
Thanks to Shadow_Streath and sweehung for this guide.