- "None of them had ever seen the boy before, or dreamed of his existence, until the day they watched him burn in their mother's arms. I can only guess that their mother had kept him secret to protect them from this exact outcome. But why had she created him in the first place? Who was he, and what had he meant to her that would cause to cross this most uncrossable of lines?"
- ―Carlisle, on Vasilii and Sasha's history.[src]
Vasilii was an immortal child created by Sasha, and the reason why both of them were killed by the Volturi. This was why it was hard for Tanya and Kate when Irina was killed by the volturi, as Sasha was their mother/creator.
Vasilii was born during the plague of the immortal children, long before the time of Carlisle Cullen. He was considered to be part of the Denali coven, even though Sasha, his creator, kept his existence a secret. Sasha had three daughters (Tanya, Kate, and Irina) who never knew of Vasilii and were innocent in the eyes of the Volturi. Vasilii was physically 3 years old when Sasha transformed him.
When the Volturi discovered his existence, they executed both him and Sasha, due to the danger he provided to the vampire community, with Tanya, Irina, and Kate as witnesses. As a result, the three sisters obeyed the laws without flaw for fear of punishment.
In Breaking Dawn - Part 2, Vasilii displayed round cheeks, blond hair and vampiric red eyes. As described by Carlisle, immortal children were so beautiful that even mature vampires would love them as soon as they see them.
His name was only revealed on the Vampire Index at the end of the Breaking Dawn novel.