Back for the Dead: Ursa, Tristan, and some of Clan Wren return in what is their third episode on Rebels, only to be killed halfway through the premiere by Sabine's Imperial weapon. Though, not really; Ursa and Tristan somehow escaped and survived, according to the season trailer.
Ezra exits a hallway with his lightsaber still ignited as the door opens behind him, entering a chaotic battle scene, in a similar shot composition as when Obi-Wan entered a war-torn Sundari in "The Lawless" in The Clone Wars.
The reason why Sabine knows quite a bit about art besides doodling despite her age (like saying she based her art off of Janyor of Bith's works when talking to Lando in "Idiot's Array") is likely related to her father being an expert in art himself.
Foreshadowing: Ursa has a portrait of herself in the Wren household, likely created by a professional formal artist. Since Sabine probably got her love for art from someone, we find out that Sabine's father is that person, also implying that he made that artpiece of his wife.
Oh, Crap!: When Sabine realizes the Empire is about to deploy one of her superweapons on her family.
Passing the Torch: Despite being reluctant at the beginning of the episode, Bo-Katan accepts the Darksaber and the mantle of leader from Sabine in the end.
Refusal of the Call: Sabine tries to give Bo-Katan the Darksaber and the responsibility of leading the Mandalorian Resistance, but she refuses, saying that she doesn't believe she can rule Mandalore as the successor to her sister out of fear of an in-universe Tough Act to Follow as well as having lost some self-confidence after the Empire took away her position as leader of Mandalore despite the Jedi entrusting her as such during the Siege. She gets better.
The Reveal: One of Sabine's Imperial weapon is something akin to a nuke.
Scream Discretion Shot: Sabine and the others only hear Clan Wren panicking over the comm when the superweapon is about to be deployed. It ends in a long beep, as the comm has been destroyed...
To Be Continued: When the episode premiered at Celebration 2017, only the first half of it was shown likely because the season is only fifteen episodes.
Trailers Always Spoil: While only the first half of the episode was shown early, the Season 4 trailer revealed that Ursa and Tristan are unharmed, the rebels and the Mandalorian Resistance head towards Sundari at night, and that Sabine and Bo-Katan get into a fight, which is eventually followed up by Bo-Katan taking up the Darksaber and becoming the leader of the Mandalorian Resistance once again.