(see TOI, p. 7)
Figures with the Ghost ability cannot be attacked by adjacent figures using a melee attack.
A figure with the Reach ability can attack a figure that has the Ghost ability using a melee attack, as long as both figures are not adjacent and the Ghost figure is within range of the Reach figure.
Heroes with the Ghost ability: Zyla.
Feats granting the Ghost ability: Skulk (1 turn only, if a surge is rolled).