The Black Garden is one of the Warfronts available in Rift Planes of Telara. This is the first Warfront you are able to participate in as a low-level player.
If you are not yet familiar with them, Warfronts are a lot like Battlegrounds from World of Warcraft. They are instanced PvP-only mini-games where teams of players (both random and arranged) queue up to do battle against the opposite faction.
By competing in Warfronts, players can earn favor (used for PvP rewards), experience points, and faction points. The Black Garden in particular offers faction points for the Caretakers (Guardians) and the Moribund (Defiant).
Once you have queued for a Warfront, you can go anywhere you want in the world and will be alerted when the queue is ready.
Once you are inside the Black Garden, the premise is fairly simple. There is a publicly displayed score for each team, and the first team to 500 points wins. Most of your points will be earned by carrying a flag, called the Fang of Regulos, which spawns in the center of the map.
Players can capture this simply by right clicking on it and after 1.5 seconds they will pick it up. Once a player is holding the flag, his or her team will begin earning points. Players can also earn points (albeit very few) for defeating other enemy players in combat.
However, there are two caveats to this. First of all, the player who is holding the flag takes damage over time. This increases considerably as time goes on, and once you pick up the flag there is no way to drop it. Eventually, you will die from damage. This prevents a single player from holding the Fang of Regulos for the entire game.
However, there is another facet to this game as well. The closer you hold the Fang of Regulos to the center of the map, the more points you get. If you run the flag far away from the center towards your base, you will only earn 1-2 points per "tick", whereas if you hold the flag in the center of the map you will earn 6 points.
Since you play to 500 points in The Black Garden, this is actually a lot of points. This encourages players to stand in the center of the map with the flag which makes the game a lot more exciting than your typical capture the flag type of PvP game.