Porter plays Tara's mother, Lettie Mae Thornton. Her character will be a regular part of the vampire drama's seventh season, which will be its last.
She has previously been billed as a special guest star for True Blood.
Porter has also appeared on several TV dramas, including two seasons on The Newsroom and roles on Grey's Anatomy, American Horror Story and Glee.
The 10-episode season will air in 2014.
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