For this achievement you'll need to recruit a Star Dragon as a High Elves faction. This can be done in both the Eye of the Vortex campaign and Mortal Empires campaign.
To recruit Star Dragons you'll need to have a lvl 5 city. Build the Dragon's Lair and upgrade it to the Dragon Keep. Take an army with at least one empty slot and recruit a Star Dragon (usually takes 3 turns when the army is in the region of the building).
When the army contains the Star Dragon, the achievement should unlock.