Tree of Whispers content will not be available until a "specific chapter" of the main questline has been completed.
The Tree of Whispers has a few loose ends to tie and needs your help settling the score. Displayed on your map, players will see various Whispers scattered throughout Sanctuary. Complete the associated task and be met with experience, gold, and Grim Favors. The number of Grim Favors you receive from completing a Whisper varies, but once you’ve collected 10 Grim Favors from the dearly departed, they can be exchanged at the Tree of Whispers for a bevy of loot and experience to reward your heroics.
The types of Whispers a player sees on their map rotate frequently, with new ones becoming available throughout the day for you to track. Each Whisper will offer both a different set of rewards and experience for completing it.