He's not dead he's just a vatboy.
Sahildar of House Tezeta is Damondar II's oldest child, only son, and only child with his wife Queen Azadeh Xhas. Unfortunuately when he was a teenager, he was aboard the Magnanimity of Dihoc when it had a misjump on the way to Altaxa. He was recovered, but ever since he has resided within the Scholar's Tower of the Floating Palace as they try to restore his health and vitality.
Despite having spent his waking life not particularly interested in politics and the last decade unconscious and in a vat, there are still many in the Floating Palace who do not want his younger sister to be named heir in his place. Part of this is because he is a man and the Sunbird is not, though much of this is likely because he has had a decade to be a half-martyr and blank slate anyone can hang their political opinions on.
Nihnsahiid of House Osada was his best friend. There may have been talks about him being betrothed to Nazanin Dhirbai, or maybe she was a love interest his parents didn't approve of?