Legendaries are in Pokemon Conquest like any other game, but they are much harder to get. Only one Warlord has a Good Link with them, and usually, they have to evolve with them. Here is the list of Legendaries, their move, their best link, their location, and the requirement for them.
Reshiram - Blue Flare - Hideyoshi - Ignis - Special Event
Terrakion - Sacred Sword-Keiji-Cragspur-Have 70% link with Bastiodon.
Articuno - Blizzard-Mitsuhide-Nixtorm-Clear Mitsuhide's episode, control Mitsuhide, and have all of Nixtorm's Facilities at Lv. 3.
Registeel - Flash Cannon-Ieyasu-Valora-Have 15+ Warlords with Best Link and the Unifier Crystal.