What the achievement asks
Complete the introduction quests to Zandalar. Ten points, account-wide, Horde only.
This is the first thing your character does in Battle for Azeroth, and it is the gate on everything else the Horde has in the expansion.
Nothing in Zandalar exists before it
Until this chain is finished, the three Zandalar zones cannot be selected, the war campaign has nowhere to start, and the world quest map is empty.
That makes this the achievement to do before any other Horde objective in Battle for Azeroth, not because it is difficult but because it is the door.
Once it is done, the zone map in Dazar'alor lets you choose one of the three Zandalar zones. You take them one at a time, and a zone you have selected must be finished before you can switch to another.
What it unlocks
Finishing the introduction opens the whole Horde half of the expansion at once:
- The Throne of Zuldazar, The Dark Heart of Nazmir and Secrets in the Sands, the three zone stories
- The exploration achievements for Zuldazar, Nazmir and Vol'dun
- World quests across both continents, which Wide World of Quests needs a hundred of
The Alliance equivalent is Come Sail Away, and neither faction can earn the other's.