Match the weapon, crystal stage and crystal color.
Most C-grade and higher weapons support a Special Ability selected from that weapon's options. The crystal color determines the option; the required stage and additional cost depend on the weapon. Do not buy a crystal based on grade alone.
Eligible non-dual S-grade weapon enhancements require a stage-13 Soul Crystal. The top A-grade additions in Interlude use stage 12, covered in the separate guide. Sealed S-grade equipment can be unsealed through the Blacksmith Of Mammon.
For an eligible A-grade weapon, the Blacksmith Of Mammon performs the enhancement. Gather the exact weapon’s ingredients and Soul Crystal, and unequip the weapon. Existing enchant levels are preserved when adding the SA. Check the Gnosis and seal-period guide before travelling to Mammon.
Write down the full weapon name, desired Special Ability, crystal color and crystal stage before shopping. Two weapons of the same grade can require different crystals or offer different abilities. Match the weapon’s own option list, then obtain the additional ingredients requested by the appropriate blacksmith. An SA choice is separate from ordinary enchant levels.
For an A-grade enhancement, arrange access to the Blacksmith Of Mammon before travelling with the materials. Unequip the intended weapon and verify the exact enhancement in his dialogue. Adding the SA preserves its current enchantment, so there is no need to rebuild an existing enchant solely because you are adding a Special Ability.