Arrow” is primed and ready to take your breath away with its season 5 midseason finale. The winter hiatus is looming, but Oliver Queen (Stephen Amell) and Team Arrow will make sure Star City isn’t far from your mind this holiday season. Set to air on Dec. 7, “What We Leave Behind” will send viewers into 2017 with at least one major shocker.

Stephen Amell talks ‘Arrow’ surprise

Midseason finales tend to stack the deck in terms of mind-blowing revelations, and none so much as those that hail from the CW’s DC universe. This time, however, even the “Arrow” cast was caught off guard by a pivotal moment from episode 9.

“When I read 509 – and this hadn’t happened in awhile – I literally went [imitates gasp] reading something that happened in act 5, I believe,” Amell teased on Facebook.

“It really took me by surprise.”

Is it a gasp of realization or one of shock? Unfortunately, the actor couldn’t say.

So, what should fans expect from “What We Leave Behind”? Check out the official summary:

“After Prometheus attacks Curtis (Echo Kellum), Oliver realizes Prometheus knows all of Team Arrow’s secret identities and is planning to come for them one by one,” The CW teased. “Felicity (Emily Bett Rickards) and Detective Malone (guest star Tyler Ritter) discover a clue that links Prometheus to Oliver’s past.”

What’s next on ‘Arrow’?

Before the Arrow can continue his war with Prometheus, the vigilantes of Star City will team up with the heroes of “The Flash,” “Supergirl,” and “Legends of Tomorrow” in a four-night crossover event. The Dominators are coming, and it is up to the Berlantiverse to band together against them.

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Kicking off on “Supergirl” this Monday, the crossover will come to “Arrow” on night 3. “Invasion!” will mark the series’ 100th episode and, as such, the hour will pay homage to Oliver’s past. After waking up in a world in which he never made that fateful trans-Pacific trip on the Queen’s Gambit, Oliver will find himself engaged to Laurel (Katie Cassidy) and surrounded by a happy, healthy family (see a preview below).

“Everything seems perfect, but Oliver starts to notice small imperfections that make him question this new reality,” The CW teased. “Meanwhile, Felicity (Emily Bett Rickards) and the recruits take on a new threat with help from The Flash (guest star Grant Gustin) and Supergirl (guest star Melissa Benoist).”

Find out more when “Arrow” airs Wednesdays at 8 p.m. ET on The CW.