Legio Audax or 'The Ember Wolves' are closely associated with the World Eaters during the Great Crusade and Heresy and turned traitor with them. They fought on Istvaan III and are a new invention of Black Library/FW but will probably be retroactively added to fight on Earth when they get round to writing that up in however many years.
The Legion is mostly Warhounds and they have a harpoon like weapon called an Ursus Claw on them. See the wiki on the legion here - http://warhammer40k.wikia.com/wiki/Legio_Audax
Legio Audax's heraldry is "dark red trimmed with iron, and decorated with embers and cracked lava designs."
This is an illustration from a Black Library book of multiple Legio Audax Warhounds trying to bring down an Imperator with their harpoons weapons (I'm curious is it was successful or not):
