Spoilers for 'Shadowhunters' season 3, episode 18 show Clary, Magnus, and Alec faced with life-altering dilemmas

Clary's darkness is catching up to her as the Shadowhunters figure out the Heavenly Fire's power. But while Magnus bonds with Maryse, Alec's encounter with Asmodeus is cruel

                            Spoilers for 'Shadowhunters' season 3, episode 18 show Clary, Magnus, and Alec faced with life-altering dilemmas

This article contains spoilers for 'Shadowhunters' season 3 episode 18 'The Beast Within'

The promo trailer for 'Shadowhunters' upcoming episode 18 'The Beast Within' was just enough to tease fans and leave us waiting eagerly in anticipation, thanks to all the several sneak peeks into the various subplots it offered.

For starters, we have Isabelle Lightwood (Emeraude Toubia) finally figuring out the secrets and capabilities of Heavenly Fire - the supernatural element that will be able to severe Clary Fray's (Katherine McNamara) life-force rune bond with her evil twin, Jonathan Morgenstern (Luke Baines.) But that will come with some definitive consequences. For starters, Clary is unnaturally worried about how the Heavenly Fire will impact Jonathan, owing to her recent revelations from his flashbacks. Clary now feels guilty for not coming to Jonathan's rescue when he was trapped and abused at Edo, something that will only be seen to intensify the impact of the life-force rune's darkness on her demeanor.

In other news, a separate promo teaser reveals that the upcoming episode will also have a sweet moment between Magnus Bane (Harry Shum Jr.) and his would-be mother-in-law, Maryse Lightwood. Magnus' boyfriend Alec Lightwood (Matthew Daddario) asks him to see Maryse (Alec's mother), to help him cope with a new life and adjusting to it. Magnus has lost his magic once again after his body rejected the new magic from Lorenzo. But this time around, he's more readily accepting of life without magic, and in his conversation with Maryse, he finds out about her evolving relationship with Luke Garroway (Isaiah Mustafa) — the former werewolf alpha.

But while we see such an accepting side of Alec's mother, not everything goes alright in Alec's encounter with Magnus' father, Asmodeus. Alec reaches out to Asmodeus to seek help regarding Magnus' current situation, but in his signature style, Asmodeus offers help but once again, there's a condition that comes with it. Only this time, it's far crueler. "Break his heart to save his life," says Magnus' father, meaning that he will only offer help if Alec and Magnus end their relationship. And while that is something exceptionally hard for the two of them to do, fans on the other side of the screen will probably be more devastated if that plan is actually put to action.

'Shadowhunters' returns with its season 3, episode 18 'The Beast Within' on Monday, April 15, at 8/7c, only on Freeform.