To enter Briarheart the first time, you must prove yourself to the commander of the Tanglewood Fort in Nettleseed. There is a warpstone inside, so you only need to do this once. There are no collection spawns in Briarheart. There is an Exploration collection, Briarwood - Briarheart. There is a Locked Treasure Chest, here, too. There is also no farm, but Rich Gold Mining can be found in plenty.