For the Black Garden, you need to capture the "Fang of Regulos" at the center of the map. Holding on to the Fang will give you points each tick, and you need to reach 500 to win. Killing enemy players will also give you on average of around 2 points per kill.
The Black Garden is a Capture The Flag type game -- with the exception that the Flag does periodic damage against you, marks you as a target against players, reduces the healing you receive, and slows your movement speed. Oh, and the points that it gives you depends on how close you are to the center of the map, from 1 point on the outskirts to 6 points at the center. But other than that, it's just your standard CTF match.
So in order to win, you need to keep in mind a few things that are around the map. For one, you can use the spires around the center to cut off LoS, or Line of Sight. Cutting off the LoS means that if any player is casting a skill or magic against you, the channel is cut off and fails.
The second major thing is that on both sides of the map, there are buffs you can pick up to boost your damage. You will find them under these small huts, so if you do decide to grab them, make sure to rush back to the center to defend your flag carrier.
The third thing is that there are walls around the map that you can access before you jump down to the pit that is the Black Garden. You can run on top of them to attack or heal from range. Melee attackers can't attack you, but instead of hiding in between your teammates you are marking yourself as a target.
Since we covered the environment, lets start talking about how you can steal the Fang.
Remember how we mentioned cutting of line of sight? Usually in this match you have the two teams in a slug-fest against each other from opposite sides of the map. But with the spires in the way, you can stand at the edge and try to pick up the Fang. Just be careful not to grab the enemy's attention while doing so.
Another way to grab the Fang is to kill the enemy player who picks it up, or pick it up from a teammate that drops it. If you do grab it, remember to keep in mind that at the center of the map you are getting around 6 points per tick. If you do not have the healing power to withstand both the damage you will get from the enemy and from the Fang, you can also pull the Fang back to the tree on your side. These trees can be used to cut line of sight, and can provide an easy looping exit in case people chase you.
Area of Effect attacks help stop the enemy from stealing the Fang, but single target attacks are better for taking down targets one by one.