Cletus Cassidy's is no longer a living being as we one them to be. No longer a he or she, it is now a hideous hive mind of slithering life—a mass of worms that clings to the vague shape of the body that granted it this new existence, and can wield the powers and magic the spellcaster had in life. It retains memories of its life as a spellcaster before its death, but is not undead—it is a hideous new form of undulant life.
When a powerful spell-caster with a strong personality, a lust for life, and a remorselessly evil soul dies and is buried in a graveyard infused with Eldridge magic, a strange phenomenon sometimes occurs. The flesh of the decaying body fats and instructs the very worms that gnaw, and these graveworms quicken not only on corruption but upon the spellcaster’s memories and magical power. The spellcaster’s very soul is consumed in this vile process, only to be split apart to inhabit each of the individual chewing worms in so many fragments.
To create one on purpose is an all-together different and difficult thing, especially when the traits of the spellcaster are different. Cassidy is the only known case of someone choosing to be a Worm that Walks. In life Cassidy didn't have the lust for life and remorselessly evil soul that a Worm that Walks is so in need of to be created. It took the Vanguard many decades of research, and several failed attempts to learn how to create a Worm that Walks on purpose, and the process died with them.
"Some of these items can help you travel through time, and others can even raise people from the dead. I implore you, do not use these Items. Time travel never has good results. You see, the present wasn't meant to go on without you, and the future was never meant to have you. And it is a universal truth that something bad always happens whenever someone is brought back from the dead. Never has someone been resurrected without terrible consequences. For those who are brought back from the grave know this, the world was meant to go on without you, and you are unwelcome. Like a body trying to push a foreign object out of it's system, the world will convulse and heave and do everything it can to remove that which shouldn't be in it's system - you." - Worm that Walks on the danger of the Items
The Worm that Walks was created by the Vanguard to eternally wait in The Vault for raiders. It was also to ascertain the raiders' purpose, and if their purpose was that of the Vanguard's - to destroy these Items with The Key, it was to help them. But if the goal of the raiders was anything else, he was to try and convince them of the need for the Items that were locked away therein, to remain locked away - discourage them by warning them of the Item's danger.
The Worm that Walks has lived for so long it has forgotten it's name, it's gender, and retains only it's strongest memories that it constantly dwells on and relives to pass the time stuck in his underground coffin.