For this achievement you'll need to build at least 5 Elven Embassies as a High Elf faction. This can be done in both the Eye of the Vortex campaign and the Mortal Empires campaign.
Elves Embassies can only be build in lvl 3 cities, but that means they can also be build in minor settlements. Occupy a few cities and settlements and upgrade those to lvl 3 when you have enough growth. Build 5 Elven Embassies (max 1 per city/settlement) and you'll get the achievment. During a normal High Elf playthrough you'll get this naturally as the Elven Embassies are part of building an income and near the end of the campaign you'll have a whole bunch of embassies.