From Battle of the Immortals Wiki
See also Fortification Mathematics...
Fortifying refers to upgrading the main attributes(stats in white text) associated with a certain piece of equipment. This can be done by going to NPC Johnny at (213,188) in Atlantis Square. Each piece of equipment specifies the maximum level that it may be fortified to. Each level of fortification lowers the chance of the next fortification succeeding. However, you may use up to 6 items to assist in Fortifying, and each item added increases the chance of it succeeding. If a fortification fails, the piece of equipment loses 1 level of fortification.
Fortifying your gear is an extremely effective way to increase the abilities of your character. Beyond just adding extra statistics to your armor and jewelry pieces, certain levels of fortification also open up new gem slots. A second gem slot will be unlocked at a fortification level of 4, a third at 7, and the 4th and final at 10.
In order to fortify armor, Fortify Crystals must be used as well as a varying amount of coins. These can be obtained either through combining 5 Fortify Crystal Shards or as drops from Chief or Lord monsters under level 75. You can also use the Major Fortify Crystals which are obtained from the item shop. These have a much higher success rate.