Unlocked at Level 15, Advanced Crafting is certainly a process that is "not for the faint of heart" or the newer players, it will take a lot of time and Sword Coins to complete these crafts.
Advanced Crafting blade in working progress.
Advanced Crafting uses 1 to 3 sword essences and two kinds of rare Materials (Any material that cost Smith Tokens{excluding Divine Metal} in the shop or can be purchased in the Black Market) to create a "Master Sword". Master swords can take a lot of work to craft and more often than not will cost a lot of Sword Coins, but completing the Collection page will net you a mysterious Cosmos Shard ore.
In order to perform an advanced craft or create "Master Swords" the following conditions must be met:
1) You must have at least a 2-star Mastery in both swords. While this may seem easy, it is still going to be difficult for the rarer materials.
2) You must have at least one Sword Essence . Sword essences are obtained by fusing together 10 Sword Dusts . This process takes half hour and does not involve hammering, as the craft does not have a quality and always succeeds, producing the same result. Sword Dusts can be obtained from the Daily Wheel, completing Weekly Quests, hammering swords, and certain bosses & enemies in Adventure. For more info on how many you can obtain per hammering see Crafting Points (CP).
3) You must have 10 of each of the materials you would like to fuse. All-in-all that is a total of up to three sword essence and 20 rare materials per each individual sword. Thus, if you wish to attempt advanced crafting without spending tokens, it will take a while.
4) Like any Fusion , in order for advanced crafting to be successful you must make sure to acheive and maintain the minimum Quality required. This is dependent upon the Master Sword Level you are doing. There are 3 levels depending on how many Sword Essences you used.