A Leviathan Pattern Siege Dreadnought is the latest thing to come out of Open Day that is going on now. Its a huge Dreadnought, with heavy armour and lots of hull points. It will get a full release sometime from Forgeworld before Christmas, and we already have its rules to see for both 40k and the Horus Heresy.
Check out the image above, its from Allies of Convenience, where they also have the full rules up.
http://allies-of-convenience.blogspot.co.uk/2015/10/leviathan-dreadnought-rules.html