This achievement requires that you recruit special character 2, Jadou.
I've seen all kinds of speculation on what the requirements are, and without a ton of testing, it's impossible to find out which are real, and which aren't. I'll just tell you what I did to make it simple. This must be new game+
First, I've seen it posted that you need to beat Jadou in the supply lines in the desert mission. I'm not sure if this is part of it, but I killed him anyway. New game plus with upgraded weapons makes this fairly easy.
I've read you need to recruit all possible characters. Break/Al as far as I know can not be recruited in the same playthough, as they need different kingdoms dead at different times. I assume this requirement refers to having recruited every other character in all playthroughs. Such was the case for me.
The Overlord army needs to be destroyed. This was the second to last kingdom I destroyed. I had Army of Flauster left along with New Muromachi. Then the story mission Hunt The Fiends should show up. Finish this mission.
Next, you'll need to increase your mercenary rank, which you can do by getting A and S ranks in missions. After you rank goes up once or twice, you should be able to get a conversation with Eunice. Talk to her and then the mission Snow Queen should appear. Finish this mission with Little Snow alive and you will recruit her. You MUST have Little Snow to get Jadou. This mission appeared when I was a High Mecenary.
Jadou's mission "Dark Royalty" showed up right after I finished Snow Queen. You need only finish Dark Royalty to recruit Jadou and get the achievement.
If something is not showing up for you, there a couple of things you can do :
Increase your mercenary rank. Some missions show up at certain ranks.
Recruit characters. There may be other character's missions and/or conversations that you need to get Snow Queen or Dark Royalty to show up. Recruit whoever you can and listen to every conversation you see, and check back often.
This is by far the stupidest achievement in this game. The triggers are a pain, and I've read about people getting Jadou at different times. Keep multiple saves.
Good luck!