Potential Spoilers Ahead!
At the end of Season 3 Episode 21, Team Flash has successfully
built the Speed Force
However, a couple of them mention that it will need a huge power source to work. As the show fades out, Barry looks (wistfully? knowingly?) off to the side and the last view that watchers get is of:
a reddish, glowing stone in some form of container, next to a menacing King Shark.
What is this, and was Barry's aforementioned look an indication to viewers that he was thinking of it? I can't recall if this is the first appearance on the show or if my memory of previous episodes is failing me. I have not seen the most recent episode of The Flash: SE3E22, Infantino Street.