Brands Fire?

In the story about Brand and Ryze, they find one of the world runes the ryze had had a missen to hide away. But Brand was lusting for power so he absorbed the rune. The thing i wanted to ask is. Why did he just become fire and not something like Frost (Like his homeland). Was it forged from his rage and then turned into some fire creature or did he have some other power within him that came out when he got the rune?