Unlock the following units by performing the tasks:

Unit           How to unlock
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Black Knight - Lose the first battle in "Tale of Sir Edward" (tutorial level).
Marine       - Lose the first battle in "Tale of Sir Edward" (tutorial level) twice.
Revenant     - Win the eighth battle in "Rise of the Necromancer"; enable the 
               debug.god.mode.enabled=true code.
Undead King  - Win the game in both campaigns.
Wight        - Lose the fourth battle in "Rise of the Necromancer".