Based on a 2005 novel, Labyrinth follows two stories, charting both the discoveries of a modern-day archaeologist and the plight of a young woman who lives through the Crusades and Cathar massacres in France.
Merlin actress McGrath will play Oriane in the mini-series, which also stars Harry Potter's Tom Felton and Downton Abbey actress Jessica Brown Findlay.
"The director Chris Smith... was all for gritty realism, blood, guts and gore," she told Digital Spy.
McGrath added that Labyrinth - which has been developed by Ridley Scott - is a marked departure from her role in the "prettified" Merlin.
"[That style] is great for that [show]," she explained. "But then to come onto this, where [the director] wants heads being chopped off and blood everywhere, it was just really cool to do something very different."
Labyrinth will air on Channel 4 in the UK.
Watch Katie McGrath discuss Labyrinth below: