I believe they all have to be under your direct control, and not under the control of a military ally like with some other quests in the game. Once that is done, the quest will appear a few turns later. The quest battle itself involves killing off three Tzeentchian(?) sorcerers on 3 corners of the map in under 30 minutes. There's a few places where enemies are either hiding in the trees or will reinforce after passing a certain trigger, but otherwise it's fairly straightforward and I found it to be one of the easier fights in the game (I did get it rather late with a lvl 30 or higher legendary lord though). Once you beat the battle, the achievement will unlock.
An aside to the achievement, winning the quest battle also unlocks Boris Ursus, both in your current campaign and as a selectable Kislev lord in future campaigns. You get an event where you can either choose to give Boris one of the 3 major cities and create an AI faction, or recruit him to your own faction as a legendary lord (I recommend recruiting him as he's a pretty powerful character).
