Amy Brenneman has a warning for Reign viewers: Look out. Mama is on the way!

The long-time Private Practice star Tweeted yesterday that she has been cast on this fun new CW drama and will soon come on board as a major character: Mary's mother!

"Yep,it's true," Brenneman wrote. "Off to a wardrobe fitting to play the badass, Marie de Guise, on @CWReign.  Watch out, @AdelaideKane, mama's coming!"

Brenneman, who has also joined the cast of HBO's The Leftovers (opposite Justin Theroux), will debut some time in early 2014 and is signed for just one episode at the moment.

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Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.