As you enter Shadowrealm, Brandon will step on an obvious trap, blocking the way back. To open it again you will need to place something heavy on the hanging plate. You will have to make use of Fireberries to light your way. Be careful though; you can only safely move between rooms 3 times before the Fireberries will burn out. If you are caught in a dark room without any light, the shadow wraiths will no longer be repelled by the light and will devour you. If you drop a Fireberry on the ground, it won't burn out and will make that room safe to go back to if you run out of berries. To open the gate to go back, Brandon will have to collect 5 large rocks to toss in the hanging dish.
If you don't want to make your own map, use this map as a reference - the highlighted path is your recommended initial path which will allow you to grab all of the rocks you need to re-open the gate.
To the north, you will find the Chasm of Everfall which has the way out on the other side. There is no way for Brandon to get across at the moment so you will have to venture deeper into Shadowrealm. At the Pantheon of Moonlight, you will find 2 wisps that promise to teach you how to float if you fix their altar.
In the cavern of twilight, you can find a coin as well as an easy to miss rock. At the volcanic river, use Darm's scroll to safely cross and collect the iron key on the other side. Make sure you don't throw away the scroll just yet; it will come in handy again later.
Use the 5 rocks to go open the gate and go back to Timbermist Woods and throw the gold coin in the well to receive the moonstone. Put the moonstone in the empty space in the altar at the Pantheon of Moonlight to free the wisps and they will activate the purple gem on Brandon's amulet allowing him to turn into a wisp. While in wisp form, you are able to illuminate a room so you don't need Fireberries to safely get around anymore. With the wisp spell you can now cross the Chasm of Everfall and enter the Faeriewood.
Summary
Collect all 5 rocks as you go through the Shadowrealm
Find the Pantheon of Moonlight and the wisps there promise to teach you to float if you finish their altar
Get the coin from the cavern of twilight
At the volcanic river, use darm's scroll to get past and collect the key ahead
Use the rocks to go back to Timbermist Woods
Throw the gold coin into the well to get the moonstone
Use the Moonstone to repair the wisp's altar at the Pantheon of Moonlight
Use the wisp power to cross the Chasm of Everfall