They are creatures of black magic, and come in a vast array of different forms: from humanoid sword-and-shield-wielding warriors to strange helmeted flying fish and swarms of purple and black bladed spiders.
It should be noted that the Knights of the Underworld appear impervious to Sonic's standard jump attack, indicating that it is the magical power of Caliburn which banishes them, as opposed to mere physical damage. The Knights can also be defeated with strong sunlight reflected off Caliburn's blade, as seen in Crystal Cave. The Knights can, once defeated, drop the Bronze Longsword item.
List of Knights of the Underworld
Soldier-Ones: The Soldier-Ones are phantoms from the underworld and the foot-soldiers of their kind. They have no bodies to speak of, but are instead given shape by the pieces of armor that they wear. The Soldier-Ones are some of the most versatile members of the Knights of Underworld, capable of wielding several types of weaponry, with their tactics and roles varying between them.
Sword Wielders (Some have wooden/Steel Shields)
Spear Wielders (Blue highlights in armor; Mostly seen charging at Sonic; some are immobile and try to stab at Sonic)
Crossbow Wielders (Purple highlights in armor)
Axe Wielders (Green highlights in armor; Some have Steel Shields)
Giant Ones: The Giant Ones are the larges and one of the most powerful creatures of the Knights of the Underworld. These massive enemies cover a big area with each swipe, and can even knock Sonic back fiercely. The Giant Ones are so huge that they can easily pick up Sonic with one hand and toss him away, causing serious damage. In line with their strength, the Giant Ones also have high physical resilience, allowing them to take heavy amounts of abuse, and takes multiple hits to defeat (number of hits needed to defeat Giant Ones decreases with the Giant Killer and Giant Killer+1 Skill):
Crawling-Ones: The Crawling-Ones are humanoid arachnid-like enemies that can crawl on walls and ceilings, allowing them to launch attacks from above, and have liquid acid dripping from their eyes. They also have razor-sharp front pincers that they use to slash with. While the Crawling-Ones are rather fast, their mobility is limited as they cannot land on flat ground.
Fish-Ones: The Fish-Ones are soldiers in the Underworld army that have been tasked scouting and monitoring enemies and comes in different types. Despite their reconnaissance role, however, the Fish-Ones can fight back when confronted, as they can fly and shoot special energy bombs from their yellow eyes. The effects of their energy bombs are determined by their type.
Eyeball-Ones: The Eyeball-Ones, also known as the "Artillery of the Underworld", can use their antennae to gather ethereal energy drifting in the air and concentrate it into a deadly beam shot directly from their specially evolved giant yellow eyes. These laser can be guarded against. The Eyeball-Ones weakness is that they cannot move much, and are vulnerable to high-speed swordplay. Also, Eyeball-Ones with pink eyes are immature and cannot throw off the energy wave.