Paladin's mission

[Gundark]

The paladin in my group has a mission. She is trying to bring to justice a former member of her order (now a blackguard). This blackguard is the general of a large army of monsters and evil humans that is set to invade our campaign world. The paladin and a few of her comrades are planning to infiltrate the evil army and try to remove this blackguard. Removing the blackguard will not stop the army. The blackguard is using a walled city as her base of operations. The army is massive and the surrounding nations are fearful that even with an alliance they maybe powerless to stop it. The campaign will be focusing on stopping this army and saving the world. This Blackguard and the paladin where once friends so the paladin is quite determined to get this blackguard. What I'm looking for is any ideas or plot twists that I can throw in there. The party is split between 10th and 11th level

 

[CCamfield]

The blackguard is having second thoughts (actual doubts, some guilt perhaps?) but cannot stop things now that they have been set in motion; attempting to halt the army's attack would see her killed by her subordinates.

Getting rid of the blackguard might see her replaced by someone even more vile, or see the army unleashed immediately before the surrounding lands can properly organize their defenses. "You sent spies to assassinate our leader, which we cannot tolerate!"

 

[Chondu]

Or the blackguard earns final redemption in some heroic sacrifice to stop the army (think Darth Vader). If the paladin is an old friend, wouldn't he see this as a more satisfying end to the campaign?

 

[Kryndal Levik]

Alternatively, the blackguard could be "possessed" and not in control of his actions. For example- he could be being controlled by an evil/cursed magic item/artifact, or possessed by the spirit of an ancient evil (or perhaps a defeated foe of the party?). This line of thinking allows for 1) the redemption of the NPC, 2) the ability to allow the party to become involved with that redemption (e.g., destroying the artifact, seeking out the source of the corruption), and 3) provides for a potentially dramatic conclusion, as the former blackguard sacrifices himself to save the party in order to atone for his misdeeds.

 

[Bihor]

The player could learn the name of the infernal patron of the blackgard and go to hell kill the demon anf free the black guard.