Warhammer: End Times - Vermintide

This weapon is obtained via the random loot table die roll after completing either the "Reikwald Forest" or "River Reik" levels contained in this DLC. Obviously, this means you're going to have to grind these levels a bit and pray to RNGesus.

However, because the weapon is for Sienna Fuegonasus, the Bright Wizard, it is beneficial to equip her purple trinket, Sienna's Hearthwind, to whatever character you're playing - if you have it, naturally. This will increase the likelyhood that items for the Bright Wizard appear in the loot table at the end of the level.

It's not much, but it's better than nothing.

The achievement will unlock once you've equipped the Bright Wizard with the dagger.