“Once Upon A Time” Season 5, Episode 21 has a strong ominous vibe about it. For one, it is titled “Last Rites.” On top of it, the promo clip voice over seems to suggest that someone other than Captain Hook (Colin O’Donoghue) is in trouble. Who dies before the sixth season of this ABC fantasy drama? Fans are guessing Robin Hood (Sean Maguire) could die but the promo seems to have a clue: "the very heart of Storybrooke." Is it Snow White or Henry?

Spoiler alert: This update contains “Once Upon A Time” Season 5 spoilers for those who have not watched “Firebird” (original ABC air date: May 1, 2016).

In “Firebird,” Emma Swan (Jennifer Morrison) and her family and friends finally leave the Underworld albeit without Captain Hook. However, Regina (Lana Parilla) was not able to split Emma’s heart in two because he has been gone too long. So Emma had to let go of Hook. This makes fans wonder: Is Colin O’Donoghue’s leaving OUAT, or is this a happy ending for the two, eventually?

In the “Once Upon A Time” Season 5, Episode 21 promo, it appears Zelena (Rebecca Mader) will have another round of fight with the heroes, even as she and Regina have actually found peace in the Underworld. It appears someone will die in this fight, and it could be a major character.

In the OUAT 5x21 clip, Emma tries to get Mr Gold a.k.a. The Dark One (Robert Carlyle) to join her team in a fight against Hades (Greg Germann). There will be a “devastating conflict” that may involve Hades, Zelena and Zelena’s baby. It may be connected to the fact that Robin could never trust Zelena even if Regina has started to show her sister some real affection.

At the end of the promo (which incidentally does not feature Captain Hook), Hades warns someone that a mythical crystal/frozen weapon is supposed to “end” and not just kill him or her. It segues to Regina, mourning someone in the cemetery. Regina looks upset but also bit numb, as opposed to Emma who looks both shocked and devastated, unwilling to accept the tragic loss again).

“The king of the Underworld enters our world, igniting a devastating conflict that someone will not survive, breaking the very heart of Storybrooke forever,” says the voice over in the “Once Upon A Time” Season 5, Episode 21 preview.

The weight given to the perceived death in the next new OUAT episode fits the likes of Snow White (Ginnifer Goodwin) and Henry Mills (Jared S. Gilmore). Either one of them is right into the “heart” of Storybrooke. Either one of them could bring up the faces that Regina and Emma made in the cemetery. Could Snow White or her grandchild Henry die?

WATCH: Hades gets help from Emma Swan against the Dark One (ABC on YouTube)

“Once Upon A Time” Season 5, Episode 21 airs on May 8 on ABC.