This achievement is for completing the House of Ballads questline. This is a faction quest which you can begin by starting the quest Song of Sir Sagrell. You can receive this quest simply by traveling to the House of Ballads, located in the northern section of Odarath.
For specific details on each quest, please see below;
Warning - This section contains massive spoilers about the questline!!!!
Once you enter the House of Ballads you will be greeted and by a fae who I was turning in a quest to from Herc in Gorhart. Once I turned in that quest, I approached Hallam the White, who will ask you to go defeat the Grave Thresh. The thresh is located nearby and puts up a bit of a fight. After you defeat it you can retrieve Sir Sagrell's ring from his body and fight some more threshes. After their defeat you will return to the House of Ballads where you will get the right to claim Sir Sagrell's seat. Basically all this does is let all the Fae now refer to you as Sir Sagrell (funny if you're playing a female character).
After completing that quest you will receive the Ballad of Bloody Bones quest. This one tasks you with "filling in" as Sagrell in the song and resurrecting a fellow Fae who Bloody Bones will kill. Just follow your quest marker to the location. You will have to fight your way to Bloody Bones who you will then speak with and subsequently fight. During the battle Sir Farrara will fall. Kill Bloody then resurrect Farrara with the potion you received from Hallam. Once he is alive again you will head to King Wenden and turn in this quest.
The next quest Two Knights and a Troll, naturally has more of the same fighting as you've come to expect. This one sends you to the Ballad Library to collect some of the ballads. Once you arrive the librarian will be under a type of curse and will tell you the ballads have been taken. You will go to retrieve 2 of them but will ultimately only get the Two Knights and a Troll ballad (in 6 parts). Anyway once you retrieve these you can return to the library and speak to the librarian. She will then have you use the Echostone to learn a counter spell to free her. Use the echo stone, then speak to her again choosing the "human" option (I forget the actual dialogue line, sorry). She will then be freed and send you back to the House of Ballads to tell the king that the Hero and the Maid ballad has been stolen by the Maid herself.
Upon your return you will learn that the entire court has been kidnapped by the Maid of Windemere. She will ultimately serve as the final boss for this quest line. For now accept the next quest titled What Lies Beneath. This quest will find you heading to a mission where you will be asked to speak with one of the brothers of the mission. Upon entering the bukhouse and heading upstairs you will see he has been murdered. Speak with some of the other brothers to eventually learn you need to investigate the well in town. You will descend the mission's well and have to search out 4 crystals. You will find 3 then confront the murdering brother for the 4th. If you pass the Persuasion check he will remain alive, otherwise kill him. Either way you will obtain the Whistle, which will appear as an item in your inventory (and totally reminds me of the flute from the original Legend of Zelda). I recommend you add this item to the radial bar as you will use it quite a bit later on. For now, just play it and follow the quest marker that appears to the final crystal. Note that the marker only appears for a few seconds (highly annoying) but you can play the Whistle as often as you need. Once you have all the crystals return to Hallam the White and the quest will be complete.
Hallam gives you the next quest called The Champions. While it is possible to fast travel away at any point, I recommend you equip a lot of health potions and keep your inventory as light as possible as there is lots of loot and fighting ahead. This quest sees you traveling to Windemere to confront the Maid. Naturally you've got a few tasks along the way, culminating in having to resurrect then murder a Fae warrior (he is easily beatable in Reckoning mode even on Hard). Enter the newly opened area after the bridge and you're in the final area.
The previous quest will now automatically complete and you'll be given the next quest, Cursed Kingdom. The area you are now in is filled with monsters (and is very gorgeously designed)but at present they are not hostile and will not attack you. After you've looted the area head inside the only house you can enter (for now) and speak with the Chancellor. He will tell you to again use your flute to find and destroy the 5 crystals blocking your entrance to the Castle. Head outside and use the flute a bunch (this is why I hotkeyed it) and find and destroy the crystals. Afterwards head back to the chancellor who will bail but first give you the key to other house in the area and have you ring the bell in the bell tower to proceed. Enter and head upstairs and pull the string, the bell will ring and you can now enter the Castle.
The final quest begins once you enter the castle titled The Hero and the Maid. This one tasks you with "rescuing" the king and court then face the maid. I say "rescue" because with the exception of the King, the other 3 people will have to be killed and serve as mini bosses. These fights are tough so I recommend using Reckoning mode whenever possible as these mini bosses always have smaller mobs helping them.
Once you mopped the floor with all of them, the king will lead you as far as he dares go then run away scared. You will be left to face the Maid alone. Make a save before you do, she is very tough. As far as a boss goes, she is actually quite simple as you can spam attack her and stop most of her attacks. The issue comes from the fact that she will summon a lot of mobs to help her (including a pesky Thresh, some sprites, etc). I made the mistake of going into Reckoning mode and killing off most of these creatures only to watch her simply revive a lot of them. Great XP but not a great strategy for a quick takedown. Focus on the Maid. Once her health is depleted she will stop you for a bit of dialogue (so don't waste your Reckoning mode on her as it will disappear during the chat). Once you are done speaking she will still be alive do attack her with everything you've got until you prompted to kill her in Reckoning Mode.
Following the battle you simply need to head back to the House of Ballads and speak with Hallam one final time. He will thank you and then note that since the King did not fight he is no longer fit to rule and will thus appoint you the new King (or Queen) of Ballads and you will net yourself this nice 20 point achievement.