After losing his wife and children to the great infection, Will lives alone in a forest killing anyone he is suspicious of being an assassin sent by Leo.
Leo had tried killing will by casting an eternal death spell, but his wife had given him a charm imbued with magic granting him eternal life causing william to be trapped in a state of undeath.
If william parts ways with the charm he will die, so he planted it within his own heart. the purple gas is the two magics reacting to each other.