A gimmicky one: Every time it kills a unit, it grows in power. But for its abilities to work, it has to sacrifice some of that power. If used without souls, the ability is fails (and still costs mana).
Ghost Knight
A suit of haunted armor, inhabited by the ghosts of its victims and seeking only to pass their suffering to others. The Ghost Knight uses the souls of its victims to power its own abilities and increase its strength.
Soulthief (Passive, does not level up): The Ghost Knight absorbs the soul of its victims into itself. Gets a boost in a random stat (move/attackspeed, damage, HP, Int/Str/Agi...) every time it makes a kill.
Harvest Souls: (Active, does not level up) Converts all corpses (except those killed by the Ghost Knight) around the Ghost Knight into souls for it to use. As they were not killed by the Ghost Knight, its grip over them is weaker, and they only provide a fraction of the stat boosts (but count a one soul when using abilities).
Hellfire (Active, drains mana and souls): The Ghost Knight is wreathed in infernal flames, fueled by its stolen souls. Deals constant damage to enemy units around the Ghost Knight and loses souls periodically (i.e. one soul every x seconds).
Torment (Active, costs a percentage of total souls): The Ghost Knight releases the souls it has stolen. Driven mad by their imprisonment, they seek only to harm its enemies. Deals damage in a radius around the Ghost Knight. The more souls used, the greater the damage.
Spite (Active, costs a percentage of total souls): Knowing no end to its suffering than to spread it to others, the Ghost Knight delights in bringing others to the grave. Sends a stream of ghosts to attack an enemy unit or hero, slowing them and obscuring their vision. The more souls used, the longer the effect.
Possess (Ultimate, active, costs a variable percentage of total souls): Sends a fraction of the trapped souls to possess an enemy unit (cost based on the unit's current HP). Control is permanent until it dies.
(Alternate ultimate) Turns a target unit into a soulhive, giving the unit a passive attack against multiple enemies (Phoenix Fire with the Spirit Tower's attack). When the unit dies, it unleashes a Torment spell (using the Ghost Knight's current soul counter to calculate damage).