Capture the Flag is a game-type available in the new Battlegrounds mode (4v4v4). It works similarly to classic capture the flag rules and each team has a home base with a "relic." To capture an enemy relic you need to go to opponents base and simply stand beside the relic and you will automatically pick it up. Run back to your base to and stand next to your relic in order to capture it (your relic MUST be there, if an enemy has taken your relic, then your team must retrieve it before you can capture the enemy relic).
A few tips for this achievement is sustainability. If you plan on being the relic runner you are going to want to use skills/pots that increase movement speed, give you armor, and restore health as you are moving back to your base. Be sure to stick with your teammates once you have a relic and being caught alone usually doesn't end well. Sneaking up to the enemy relic can also be effective, but be sure to manage your stamina so you have plenty for the run back.