Darius had been close to fire for his entire life.
When he was young, he was notorious for running around causing mayhem. He always enjoyed visiting the palace forges to pull the beards of the blacksmiths and play with their fire.
There was a time when said blacksmiths wanted to take him as their disciple, realizing his love for fire, but those plans fell through.
At the end of the day, Darius was a member of the Void Clan. He was meant to use the main techniques of the palace.
From the day he started learning about space and time, his connection to fire began to fade, but he had several small experiences that seemed to hint towards his affinity.
And when Damien arrived, he was able to fully bloom, realizing his connection with fire and embracing it.
Damien's arrival was a breath of fresh air for everyone in the palace, not just Darius, but it was to another degree for the youngest of the Void sons.