The name of achivment when you finish the quest is "unleash the beast", and you get the outfit at the end of quest
second wikia the first addon can be made with the iten
skull of the beast that make sense since look like the headpiece of the complete outfit showed in site.
this should be the item needed for the second part of the addon, i belive both itens are rare drops of the new bosses of the soul of war quest (probabily follow the same pattern of zelos that give the outfit completing and the addons can be obtained doing final boss and mini bosses)
2 teams have done the quest in my server and still none of them drop either mount or outfit itens, more pep will been tryng and i come back to comment if there is any news