The film will mark the directorial debut of Peter Briggs, who was one of the writers on Guillermo del Toro's Hellboy and has been a writer-for-hire for studio films.
The $25m project will shoot in the second quarter of 2009.
Set in 122 AD, the story follows a disgraced Roman war hero who comes face to face with a supernatural beast causing horrendous deaths.
Mortis Rex is Latin for 'king of death.'
Los Angeles-based Stuart Pollok will produce with Matthew Dench of The Dench Arnold Agency and Marisa Kagan of Magenta Management and Production.
In addition to Intandem, the other executive producer is Dominic Ianno, founder of Blowtorch Entertainment Company.
Briggs said: 'As a former cinematographer, and as a screenwriter writing on some of the most memorable Hollywood tent-pole horror and sci-fi franchises of recent years, I've used that to shape Mortis Rex into a highly accessible piece of entertainment: by turns both an action-packed thrill-ride and equally a scary, heart stopping nail-biter. Working with Intandem and their expertise in film finance and sales I'm excited about the potential of the film.'
Intandem director Andrew Brown said: 'This is a genre that has opened up enormously in recent years through the outstanding box office success of such genre-led films namely, 300, Beowulf and Cloverfield. Peter is an exceptional writer who has created an amazing but terrifying world based on true fact and we are most confident that he will direct a superb hi-end, genre-led live-action film that I'm sure will have worldwide commercial appeal.'