Light Generals:
Oak
Sidna
Light Assassins:
Rook
Regulus
Dark Generals:
Queen of Thorns
Erebus
Dark Assassins:
Unclean Beast
Torchbearer
To summarize the assassins:
Unclean Beast: Fast melee attacker with lots of damage over time and area effect attacks.
Torchbearer: Mage character with fire and ice attacks. He has more skills than normal, which are split over two stances.
Rook: Very slow but with high damage. He plays pretty unusually, since he is a walking building.
Regulus: An angel sniper. Very good ranged hero with some interesting tricks (tracking device, mines, etc)