Things will heat up for the citizens of Storybrooke when Once Upon a Time returns to ABC in the spring. Not only will the group find themselves in the Underworld, but they'll also come face to face with the king of that realm himself, Hades.

The series' showrunners recently confirmed the appearance of the fiery one in the series, and announced that Ally McBeal actor Greg Germann would step into the shoes of the ruler of the Underworld.

In the season finale, Emma Swan (Jennifer Morrison) and friends began to mourn the loss of Captain Hook (Colin O'Donoghue), but they're not going to let him die without a fight: they summon the boat that will take them to the Underworld to retrieve Hook and bring him back to Storybrooke.

Now, though, it seems that not only will they have to face enemies from their past who have died — such as Cora (Barbara Hershey), Cruella de Ville (Victoria Smurfit) and Peter Pan (Robbie Kay) — but they'll also confront the king of the Underworld, Hades.

"We're going to be meeting the Once Upon a Time version of Hades in the second half of the season," said executive producer Adam Horowitz to Entertainment Weekly. "While he is both inspired by a myth and by the Disney movie version, as we like to do, we're trying to create our own Once Upon a Time spin. We can't wait for everyone to see what [Germann] does with the character."

So, who is Hades and what is his link to Storybrooke? Executive producer Edward Kitsis teased that the character has a "surprising connection" to one of the cast members. However, Horowitz added that he isn't actually related to anyone on the series.

Horowitz explained that the version of the Underworld that fans will see on Once Upon a Time will not include just villains, though, and that the place isn't exactly a typical depiction of hell, describing it as "a place that's filled with people with unfinished business."

The second half of the season will also see the cast facing their past and dealing with their own unfinished business.

"I think there's a lot of unfinished business and the need for emotional closure with people who have passed in the lives of our heroes, and this second half is really going to get to honor that," said Kitsis to Entertainment Tonight. "The 100th episode is the spring premiere and we've been on for five years and for us in this second half our real goal was to have a very season one vibe and I think that will be apparent after people see the premiere in spring."

Once Upon A Time returns to ABC on March 16.