RottenBoy

Officially, in the manga it hasn't been stated that Beldaruit has a medical condition, or that he was ever paralyzed. But then Chapter 81 gave us this page...

...and everything fell into place!

«My bones have been brittle from the day I was born»

Yes, this is a real medical condition.

Osteogenesis imperfecta (OI), also known as Brittle bone disease, Lobstein syndrome, fragilitas ossium or Vrolik disease.

● ICD-11: LD24.K0   ICD-10: Q78.0

«Osteogenesis imperfecta (OI) literally means “imperfectly formed bones”, and is a rare and complex genetic disorder that is often characterized by bones that break easily. The vast majority of OI is caused by variations (mutations) in genes that affect the structure of type I collagen or genes whose proteins interact directly with type I collagen.»

In other words, a person with this condition is born with bones so fragile that their legs can break under their own body weight, and even lifting an object can result in a broken arm.

This would explain why Beldaruit is confined to the chair and prefers to travel around the Great Hall through smoke illusions. If he tried to stand on his own feet, his legs would most likely break under his own weight.

We only see Beldaruit lying in bed, where he has already developed pressure sores and ulcers. (I absolutely love the level of details in this manga and how seriously Kamome Shirahama approaches depicting different medical conditions.)

Also, notice how gentle all of his movements are. He doesn't even fully close his fingers around the rope, and the way he holds his pen is so delicate:

Symptoms found in various types of OI include:

whites of the eye (sclerae) that are blue instead

short stature

loose joints

hearing loss

breathing problems

problems with the teeth (dentinogenesis imperfecta)

Potentially life-threatening complications, all of which become more common in more severe OI, include: tearing (dissection) of the major arteries, such as the aorta; pulmonary valve insufficiency secondary to distortion of the ribcage;and basilar invagination.

More information

There are several types of osteogenesis imperfecta, ranging from mild to severe. I'll only talk about Type I, because I believe that's the one Beldaruit has. His body shows no obvious skeletal deformities, and he's quite tall, both of which are consistent with the mildest form of the condition.

You can read about the other types here

"Type I OI: people with Type I OI, the mildest and most common form, may have only a handful of fractures or as many as several dozen fractures in a lifetime. They may have few obvious signs of the disorder. In Type I, there usually is little to no bone deformity. Height is less affected than in other types of OI, and many people with Type I are often similar in height to other family members. Muscle weakness, joint laxity, and flat feet are common. Dislocations and sprains may occur, as well as fractures. Life expectancy appears to be average."

Works featuring characters with the same condition: