In the Grand Campaign, when the Chaos enters the world, destroy several armies. Other factions will really, really like that you attack and defeat Chaos units. After several turns I got a LOT of invites for peace treaties, trade agreements and military alliances. Keep accepting these, as that also boosts your friendliness. After getting 5 military alliance invites this achievement should unlock.
I was playing Dwarves at that time, but my friend was Vampires. We both got all those invites from the AI's and both got the achievement during our Grand Campaign
