Mount & Blade: Warband

Conquering Calradia as ruler of your own faction will not trigger this achievement. If you want to avoid having to do a second playthrough, I recommend keeping a separate savefile of your full conquest playthrough just as you conquer the last holding of the next to last enemy faction. Replaying from this point quickly find the king of the last remaining faction and swear an oath of fealty to him (you have to talk to him personally for that, sending an emissary will eventually require the same). Now all of your own kingdom's holdings switch over to him, allowing you to sit out the time until the faction you just annihilated before registers as "dead", thereby making your new king's kingdom victorious.