'Arrow' Veteran Susanna Thompson Has a Surprise Role in "Crisis on Earth-X"

"Crisis on Earth-X" brought characters from all across the Arrowverse together, and it snuck in a pretty surprising cameo along the way.

Spoilers for "Crisis on Earth-X" below!

The fourth hour of this year's crossover saw the Earth-X villains bring out the big guns, including their own version of Legends of Tomorrow's Waverider. Kara Zor-El/Overgirl (Melissa Benoist) and Oliver Queen/Dark Arrow (Stephen Amell) were shown inside the Waverider, as Overgirl attempted to recuperate.

They were soon approached by their Waverider's version of Gideon, the ship's artificial intelligence system. And Gideon's voice was one that Arrowverse fans are sure to recognize -- as it belonged to Arrow alum Susanna Thompson.

As Arrow fans will remember, Thompson played a core part in the show's first two seasons, as Oliver's mother Moira Queen. Moira died a shocking death at the hands of Slade Wilson/Deathstroke -- one that seemed to have a significant impact on Oliver as the show went on.

In a way, Thompson voicing Earth-X's version of Gideon speaks to Earth-X Oliver's character in a way. While Earth-X Oliver has a very clear, surprising role in his planet's tyranny, his personal attachments seem to be some sort of a weakness. His feelings for Overgirl have already proved to be a liability for him, and it appears that the memory of his mother has bled over as well.

This also marks the second crossover in a row that Thompson has had some sort of role in, after reprising her role as Moira in the dream world of Arrow's 100th episode.

"Crisis on Earth-X" concludes tonight with episodes of The CW's The Flash and Legends of Tomorrow beginning at 8/7c.