Exaggerated: "The gates of Hell have opened...and its infernal soldiers are marching out. Hear them, crying out of the blood of mankind, behold their lust for war!. Go forth, and oblige their wishes! Spill their wicked blood, pierce their twisted hearts, and send them back to the abyss from whence they came!"
Downplayed: The Crimson King can only field a platoon of demons, who act as his Special Forces unit. The rest of the army is just evil mooks.
Justified: Can't stage a Demonic Invasion without soldiers, can you?
Zig Zagged: They're not demons, actually. Very much Hell's standing army, though.
Averted: The Crimson King doesn't really have any army, instead using guile and evil spells to manipulate human kingdoms into fighting each other.
Enforced: "We need a totally unsympathetic army, one that no viewer could take offence at. Oh, I know!"
Lampshaded: "To save the world, Siege, you must travel there, there, and ... there. One of the armies you'll encounter will, of course, be the Legions of Hell themselves."
Invoked: Siege doesn't know what the army on the horizon is, but loading the catapults with flasks of holy water seems a safe bet if the enemy is called the Crimson King.
Exploited: Another villian opens the gates of Hell, releasing the Legion to distract the heros while he completes his own evil plan uninhibited.
Siege and his allies do everything in their power to seal the gates of Hell so that the Crimson Legion cannot march.
The Crimson King knows that Siege is well-equipped to fight demons, and therefore raises an army from some other source to catch him by surprise.
Discussed: "Any thoughts on what we should load our catapults with?" — "Well, it could be the Legions of Hell. In that case, we'd need holy water. On the other hand, that won't do much against a plain old human army." (This counts as discussing the trope whether or not the Legions of Hell turn up, or even exist.)
Conversed: "So did you watch Bladeheart last night?" — "Ayup. That Crimson Legion was totally unoriginal, though." — "Hell, yes. Even calling it the Crimson Army would have been an improvement." — "What, and have people think you're secretly commenting on the Russian Revolution? Anyway, back to the matter at hand. How are we going to get the sausages to Wilma's wedding?"
Deconstructed: Because Religion Is Magic, Siege merely puts together an entire army of priests. When they pour Holy water all over the opposing forces, the Crimson Legion doesn't seem like a very useful army...
Reconstructed: ...or maybe, religion isn't magic. The demons laugh at the plain water being thrown at them and massacre their enemies