You need 9 adamant ingots to forge all 3 of the Legendary weapons (3 ingots for each).
According to the "Fire Amid the Water" quest text, ingots can be found in archives and repositories in the Oikos of Atlantis region, or alongside Keeper's Insights in the Porimos Ring region. They are in chests. To make them easier to find, stay outside and scout the area inside the entrance with Ikaros to mark enemies and special items.
Locations of All 9 Adamant Ingots*** Spoiler - click to reveal ***Insight of Blessings
Insight of War
Insight of the Titan
Horus Repository
Repository of Iris
The Consus Archive
Archive of Hebe
The Tinia Archive
Archive of Hephaistos
Some locations require Isu Knowledge Level 1 or more to enter.
The Forge of Atlantis is where you need to make your weapons. It is located at the gold ? symbol on the southern part of the central section, Royal Mountain of Poseidon. The ? will be replaced by a special icon fast travel point after you discover it.
When you have 3 or more ingots you can forge a weapon at the Forge by interacting [cn_Y] with the molds in the forge's upper section. Choose one of the 3 classes (Warrior, Hunter, Assassin) to begin the process.
Your reward's properties will be based on your choices. You can only forge 3 of the 9 possible weapon combinations, so choose what best suits your play style.
The trophy should pop as soon as you start forging your 3rd weapon. If not, Kennyannydenny says, make sure you have all 3 weapons in your inventory and not in storage.
All Isu Forged Weapon/Class Stats
*** Spoiler - click to reveal ***Isu-Forged Assassin's Blade
+25% Assassin Damage
+30% Damage with Daggers
Reduce all Cooldowns by 10% on Assassin Ability Kill
Isu-Forged Warrior's Blade
+25% Warrior Damage
+30% Damage with Daggers
Reduce all Cooldowns by 10% on Warrior Ability Kill
Isu-Forged Hunter's Blade
+25% Hunter Damage
+30% Damage with Daggers
Reduce all Cooldowns by 10% on Hunter Ability Kill
Isu-Forged Assassin's Mace
+25% Assassin Damage
+30% Damage with Heavy Blunt Weapons
Assassin Damage applies to Nearby Enemies
Isu-Forged Warrior's Mace
+25% Warrior Damage
+30% Damage with Heavy Blunt Weapons
Melee Attack Damage applies to Nearby Enemies
Isu-Forged Hunter's Mace
+25% Hunter Damage
+30% Damage with Heavy Blunt Weapons
Basic Arrows Damage applies to Nearby Enemies
Isu-Forged Assassin's Spear
+25% Assassin Damage
+30% Damage with Spears
Convert 50% Assassin Damage bonuses to All Damage
Isu-Forged Warrior's Spear
+25% Warrior Damage
+30% Damage with Spears
Convert 50% Warrior Damage bonuses to All Damage
Isu-Forged Hunter's Spear
+25% Hunter Damage
+30% Damage with Spears
Convert 50% Hunter Damage bonuses to All Damage