Not to be confused with the Witch's Broom, the Expert-exclusive flying mount also dropped by the Mourning Wood.
The Hexxed Branch is a mount-summoning item which summons a rideable Tree Mount. It is dropped by the Mourning Wood in Master Mode with a 25*1/4 (25%) chance. The Tree Mount is a ground-based mount; it is unable to fly. While travelling at full speed, enemies in its path will be hit by 120 base summon damage.
The Tree Mount's acceleration is extraordinarily high. It achieves high speed very quickly without a dash (e.g. Tabi or Shield of Cthulhu) and can allow one to make sharp turns midair if it is summoned while falling, making it rather useful for boss fights.
The buff's tooltip "Run, forest, run!" is a pun based on a famous line from the 1994 American movie Forrest Gump.
The Tree Mount is the fastest ground-based mount in the game. It is as fast as the fastest flying mount, the Shrimpy Truffle while it is in water, despite the fact that in real life trees rarely move at all.