Based off of the Greenscale's Blight raid, The Blighted Antechamber is a Capture the Objectives type map. To win you have to reach 1,000 points, using either the enemy flag or the three different objectives.
The full version strategy of the map is that both teams have a flag that can be captured and turned in on the enemy side for points. The fastest way to win this match is to capture the enemy flag and repeatedly turn it in. Failing that, you need to aim at grabbing the objectives, and slowly gain points to reach 1,000.
With a 15 minute time limit, most victories happen by combining both objectives, with the flags being gridlocked. A gridlock happens when both teams are holding the other's flag, and unless you can kill the enemy flag carrier and recover your flag, you are forced to rely on the 3 points.
In this match, classes and builds that can increase movement speed or stealth excel. For instance, bard specs have an AOE movement speed buff, which helps with the capture-the-flag train. Also, since there is a lot of movement for this match, stealth users who can hide at points until the coast is clear can capture points that aren't otherwise capturable. But be careful of the teleports littered over the map. If you are stealthed and stand on one of them, you will drop the stealth and be transported.
A cool about stealth players however, is that if you watch the characters to see what direction they are facing and sneak behind them -- even if you unstealth -- since you didn't pass their line of sight they won't expect you to be behind them. So if you are stealthed and unnoticed, you can stand in the middle of a group of enemy players and steal the flag with them none the wiser. Just be prepared to run like crazy.
Remember to communicate with your team from the beginning, and do not throw all of your eggs into one basket. So ask who wants to stay behind and protect your flag, who wants to form the flag running team, and who wants to aim to capture objectives.
If you are holding the flag, you can't teleport using abilities or with the teleports in the map. So the best idea is to run back on the outside of the map, hopefully through the objective you have. If you are being chased, run to the closest teleportation device. While you can't teleport - any melee characters standing next to you will.
Another location you can't go to is the top of the spire, which is the middle objective of the map. What happens if you do have the flag is that a wall will pop up and prevent you from entering. What most people don't realize about this wall is that while it does affect you, it also prevents everyone else from entering or exiting the point.
So if the enemy team is centered around the spire and if you have the flag, then consider climbing up near the top and locking that team in. You will basically be winning through points from the two objectives outside. But be prepared to get lots of hate mail from the opposing team!
The best way to protect your flag is to use a few high CC (crowd control) and HP characters. The idea is to be able to prevent a large amount of people from casting the pickup skill, and if you are holding the post solo you can sustain with your high health until backups from your team can come help.
So an ideal team setup for a 15 man group is: