For this achievement you have to complete all the quests in Cyrodiil. All the quest are found in different villages across Cyrodiil. For the quests to unlock in those villages you first have to find a quest random across Cyrodiil. If you find these quests they ask you to travel to one of the following locations:
- Bruma
- Cheydinhal
- Chorrol
- Cropsford
- Vlastarus
As suggested in the comments it indeed is risky to travel in enemy territory because there is a change that enemies will kill you. If you are close to enemies or an enemy keep just sneak past them or stay hidden for a while untill it is save to travel again.
Here a screenshot with the village locations:
For the achievement to unlock you have to complete a few ingame achievements. For each ingame achievement you have to complete the following quests.
Bruma Adventurer
- Dangerously Low
- Capstone Caps
- Lost and Alone
- The Standing Stones
- Enemy Reinforcements
- Know Thy Enemy
- Requests for Aid
- Bring Down the Magister
- The Unseen
- Timely Intervention
Cheydinhal Adventurer
- Thorns in Our Side
- Spice
- Prisoners of War
- The Burned Estate
- Ayleid Treasure
- Bloodied Waters
- Keepsake
- A Debt Come Due
- Stacking the Odds
- For Piety’s Sake
Chorrol Adventurer
- Death to the Black Daggers!
- Guard Work is Never Done
- Field of Fire
- The High Cost of Lying
- The Cache
- Abominations
- Claw of Akatosh
- Overdue Supplies
- The Lich
- Black Dagger Supplies
Cropsford Adventurer
- Seeds of Hope
- Offerings to Zenithar
- Harvest Time
- The Dead of Culotte
- Crown Point
- Bloody Hand Spies
- Hedoran Estate
- Securing Knowledge
- Goblin's Delight
- Timberscar Troubles
Vlastarus Adventurer
- The Direct Approach
- Death to the Crone
- An Evil Presence
- Bear Essentials
- Mementos
- Rock Bone Diplomacy
- Stolen Banner
- For a Friend
- Essence of Flame
- Silver Scales