While the illusion of Black Night intimidates cough the enemy, the real one grabs the newly delivered sword and goes on the attack. But the blade turns out to be a mere illusion. The sword they're defending is a decoy.
Artist note: I make no secret that I find Black Night's design amusing. I see that masked head and I just want to pat it. So when the players decided to involve an illusion in the battle, I took the opportunity to draw the illusion as silly and cute while the real one gets to be serious.
The sword being a decoy was a surprise to the players. They thought they were defending the real thing. The GM decided to lower the stakes a little, because he thought there was a real chance the players could lose this battle. It turned out to be rather stacked against them.