To obtain this achievement you must acquire an item that has the the title "Legacy" attached to whatever else name the item will carry. You can find a legacy dropped from monsters though it is extremely rare or through the randomized item pots that you find in dungeons or buy in towns.
*Through some digging with friends and exchanging with players I can verify that you must obtain these legacy items through the random pots yourself, you cannot trade and have 5 on you at a time.*
**FrogInTheStreet ha stated you can also buy the legacies from the premium shops. Albert would be one of the NPC's who sell Legacies. They do however cost life tokens, which you can buy buy if you've run out for a small amount of MSP.**
If there is any issue with the solution please tell me and ill be happy to fix it.