Ronin had told his father and Donna that he would be travelling to Scirocco for the summer. Which was not really a lie, as he had to make a stop for the potion there.
It was only in Scirocco that the Potion of Illuminance can be found and distributed.
The Potion of Illuminance was the one that humans must take in order to survive in the darkness and poisonous rancid air of the Abyss. They were believed to had been made from a strand of Aurion's hair itself, which was equivalent to a sunbeam too.
In order to survive the Sunless Realm, you need a piece of the sun within you.
However...
Would Ronin really need it?
After all, he was the incarnation of the Lord of Darkness himself, just stuck in a mortal human flesh.
But Ronin was only planning to take it just in case, and to be an alibi. After going to Scirocco, he will go for the Edge at the Kingdom of Welkin, and ride a Griffon from there to descend into the Abyss.
