Legend has it that the Tree of Souls belong to a race of giants that would embed the skulls of vanguished enemies into the trees themselves. They are totems of death that are displayed proudly by these giants and are littered throughout their lands. These models are included with the Wargamer Tree Set.
The power of a Tree of Souls lies within the posession of the skulls. If a character is able to harvest a few skull, then the character will gain the power of that specific warrior for a brief amount of time.