Following on from the events of previous episode Wrath of Savitar, the clip sees titular hero Barry Allen attempt to retrieve Wally West (aka Kid Flash) from the Speed Force by venturing into it himself. In the process, he encounters what look to be spectres of the aforementioned trio, as they warn him of the dangers of consistently tampering with things outside of his control. A particularly nasty-looking Time Wraith and Jay Garrick also make an appearance.

Eddie Thawne in The Flash
Rick Cosnett's Eddie Thawne returns as a ghost to warn The Flash about tampering with things he can't controlThe CW

In The Flash's first season, Thawne worked as Joe West's partner and went on to date and get engaged to his colleague's daughter, Iris. After some time, the gang trusted Eddie enough to let him in on The Flash's secret identity and he became a regular ally of Barry and the rest of his team (despite Barry's ongoing affections for Iris). However, Eddie's crime-fighting days came to an end when he discovered that villain Reverse-Flash (aka Eobard Thawne) was his time-travelling distant descendant and he sacrificed himself so that the yellow speedster never existed in the first place.
Robbie Amell in The FlashIn season one, Raymond was a structural engineer who worked with fiancée Caitlin Snow (Danielle Panabaker) and her employers at S.T.A.R Labs to create the infamous particle accelerator which turned Barry into the Scarlet Speedster.

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When the accelerator exploded (and turned several Central City residents into meta-humans), he became one half of the conjoint flame-producing hero Firestorm. After learning to harness his powers alongside other half Professor Martin Stein (Victor Garber), Ronnie went on to marry Caitlin and work with The Flash to fight crime. But before long, he was killed when he too sacrificed himself trying to save the city from a black hole, which means it's pretty unclear as to what capacity Amell is going to return to the series.
Despite him being a baddie on The Flash, Snart (aka Captain Cold) went on to adopt more of a heroic persona in fellow Arrowverse show Legends of Tomorrow. The pattern of characters sacrificing themselves continues, as he too died in the first season's finale saving Sara Lance, Mick Rory and more of his friends.
The trio of characters will be making their hotly-anticipated comebacks when the episode – entitled Into The Speed Force airs in the US on 14 March. The same episode will air in the UK on 21 March on Sky 1.