Roxas is again sent to Beast's Castle to defeat yet another Heartless. After making his way through the castle to the courtyard, Roxas hears a roar from outside the gates. As he makes his way to the middle of the courtyard Beast is suddenly thrown from behind the gate and lands in front of Roxas. As he enters the bridge through the gate; Roxas is attacked by his target the Infernal Engine. After defeating the Heartless, Roxas learns of love—through the Beast's unyielding protection for Belle.
The Infernal Engine is a large, box-shaped, apparently mechanical Heartless with four black wheels, two on either side of its body. It appears to be predominantly made of wood. Its face takes up its entire front side, with a large, red, jagged mouth with yellow "lips" on its lower half, and a black "forehead" with silver, metal additives on it. Its eyes are a glowing yellow color and its Heartless emblem is in the middle of its face. There is a wooden guardrail on its top decorated by yellow and red patterns as well as four white spikes. The Infernal Engine is able to release a battering ram, hammer and boxes from its mouth.
Riding atop of the Infernal Engine are several smaller Heartless, referred to as "archers" in the Journal. Each of these Heartless wears silver armor, including a bullet-shaped helmet with an opening for their black faces and glowing yellow eyes. All of their helmets sport two short spikes and a curled, black antenna. They have small legs which allow them to move around the top of engine. They wield large, indigo weapons that fire projectiles and each sports a Heartless emblem emblazoned on their chests.
The Infernal Engine's name has several potential meanings. "Infernal" could refer to the fireballs its fires, as "infernal" can mean "of a great fire", or it could refer to its antagonistic nature, since infernal can also mean "fiendish" or "of or relating to hell". "Engine" is a reference to its apparent mechanical nature and it's use as a vehicle by the smaller Heartless archers. The Infernal Engine's Japanese name, "Demon's Fortress", reflects its massive, sturdy construct and the way the archers use it as a "fortress" of sorts. This name and the Infernal Engine's behavior are also reminiscent of the Demon Wall enemy from the Final Fantasy series.