Treasure Chests and other Special Chests (like those found in Temples and Guilds) are littered around Templehelm. At the time of writing this there are over 200 to be found.
To see how many treasures you have found: /tchest count
To see how many treasures someone else has found: /tchest count <player>
List all treasure locations that you have found /tchest list <page>
When you open a Chest of this kind, you only have one chance to take the items from it. If your inventory is full, then you will not be able to take the items. If you have found a chest in a strange location, then it's probably a good idea to drop some items, or use an Enderchest before opening it.