Quick answer: The Threadbound Great Tree bell is in the Mirrorwood Depths of Crimson Forest. It is a short walk past the blossoming dead tree, which you reach by solving the mist portal puzzle.
Where the Threadbound Great Tree bell is
Threadbound Great Tree is a bell in the Mirrorwood Depths part of Crimson Forest. The bell is a wooden post with a bell hanging from it, on a misty slope dotted with blue flowers and patches of snow. Beyond the field, a massive gnarled tree bears threads across its limbs beneath a crimson-lit sky. Opening the bell interface displays the name “Threadbound Great Tree” at lower left.
How to reach it from the Mirrorwood Depths bell
The route starts at the Mirrorwood Depths bell. You have to clear the mist puzzle before the path to Threadbound Great Tree opens.
What the bell gives you
Ringing the bell opens a menu with six options: Traverse, Upgrade Equipment, Store, Train Abilities, Save and Leave. “Threadbound Great Tree” is shown in the bottom-left corner of this menu.
Resting here also autosaves the game. The save list then shows entries named “Threadbound Great Tree”. The bell is added as a Traverse destination too, so you can travel back to it from other bells.
For the Freezing Chaos weapon, the weapons list identifies Threadbound Great Tree as the nearest bell. The accessories list gives the same nearest bell for the Chaos Ring accessory, found in a chest.
Rest before you walk on
Beyond the bell, the route crosses blue and white flowers on its way to the threaded tree. After about 40 metres, stepping into the fog triggers a boss scene, and the fight starts immediately afterward. Ring the bell at Threadbound Great Tree and prepare your gear before heading there.
