Bugs are gross. Sure, some like insects (especially those scientists that have made the study of bugs their life's work), but most people tend to find insects to be disgusting. Crawling, slimy things that dart out from the dark and crawl across your toes and up your legs. It's easy to see why bugs are present in many horror movies and other works of the genre: insects are an natural, easy way to up the horror and the creep factor. Because, again, bugs are gross.
Despite this, there aren't a lot of insects in the Castlevania series, especially not in a leading, boss role. Occasionally there will be spiders or little worms that crawl around, but these are minor annoyances, not major foes. It's harder to find many instances of giant insects terrorizing the heroes of the series. That is, at least until Castlevania: Lament of Innocence came around. Then, finally, we had a proper, gross insect to act as a foe for the protagonists.
The undead parasite is, as the name suggest, the reanimated corpse of a giant, ,dead worm. This beast, resembling a giant tapeworm, crawls through the tunnels under Walter's castle, waiting for a foolish hero to come around and be devoured. It's not a monster from mythology or classic films but one of biology, a body-horror creation designs to terrify and devour. And, when found, it gives the game the dread it needed.
Castlevania: Lament of Innocence
Having just entered the castle on his quest to save his lost beloved, Sara Trantoul, hero Leon Belmont had to explore the five main dungeons of Castle Bernhard, each locking access to Walter's final chamber. The first step on his quest was venturing through the House of Sacred Remains, the cathedral built within the castle. This was a series of rooms and halls, with libraries, training rooms, and crypts beneath.
Deep down, in the darkest parts of the cathedral, Leon found a chamber filled with grime and ooze, which lead to a series of deep tunnels below. Before he could step into those tunnels (if he'd even want to), Leon heard a rumbling. A great beast emerged from the pit in the floor, a horrible, undead worm with one thing on its mind: eating Leon. As it crawled around it dropped foul, oozing beings, the soulless corpses of his many victims. Leon had to defend himself and fight the soulless, as well as the worm itself, all so he could clear this horror. Only then was the first of the castle's locks opened, granting Leon further access within.
Fighting Against the Undead Parasite:
When first entering the chamber of the Undead Parasite, Leon will see four large, creepy eyes set into the floor. These eyes have to be destroyed before the giant worm can even be damaged. These will take a number of hits to kill, and all the while you'll have to deal with the soulless that will rise from the ground, and the worm itself. It'll crawl between the gates in the wall, using the tunnels to get around the room. And, sometimes, it will also hang out in one gate, waving his head while it spews fire across the room. Leon needs to avoid these attacks while layering all the damage he can on the four eyes.
Once all four eyes are destroyed, four beams of light will shoot up from their sockets. Then a large yellow beam of electricity will shoot down to the central socket of the room. This will summon the core of the parasite, and then the true fight is on. The core will move from one socket to another, forcing Leon to dash over and deal damage, all the while continuing to dodge the soulless and the attacks of the worm. It's a long fight, made worse by the core having two life bars even once its exposed, but it's not all that difficult.