Their first encounter could be really intense. Hades might be sitting on his throne, all imposing, and Will would be a bit scared but also curious. Hades could use his powers to make the area around him seem even more menacing, like making the flames around his throne burn brighter.
Maybe Will stumbles into Hades' chamber while exploring the Underworld. Hades is initially angry at the intrusion but then becomes intrigued by Will. Will, on the other hand, is in awe of Hades' power and appearance. They start talking, with Hades asking Will who he is and what he's doing in his domain.
It could be that Will is brought before Hades as a prisoner. Hades looks down at him, sizing him up. Will tries to be brave and defies Hades at first. But then Hades starts to question Will in a more calm way, and Will realizes that Hades is not just a brute, but a complex being with his own motives and emotions.