Enter the Bibberbang field in the Underdark
Find Baelen trapped near the explosives
Use Mage Hand, Telekinesis, or a thrown item to move the backpack to him
Alternatively, shoot or destroy the bibberbangs from a safe distance before approaching
Avoid stepping on the mushrooms
Keep your party spread out
If needed, use Misty Step, Jump, or Feather Fall to reach safer ground
Speak to Baelen after he is freed
Escort him back to Derryth Bonecloak in the Myconid Colony