Those three things don’t really go together normally. But hear me out, please.
Early in the game (level 5 or so), give the characters access to an “Odd Rusted <weapon>” or “Odd Cracked <weapon>”. It seems worthless, but the description suggests that it can be repaired. Thus begins a series of upgrades where, every 5 levels or so, you can go somewhere and get something or gain access to an area useful to upgrade the weapon so that it keeps up with the player.
The tutorial part comes in when you see where this leads you. It can be used to lead players to new zones fitting to their level range, as well as to challenges that make sure they’ve mastered the basic (and even advanced) concepts of the game. It can show new players around, without being too much of a burden to more experienced players. The reward for this will be the upgrading of the weapon to the next level.
The legendary part comes in at the end. When the weapon is finished it should be, stat-wise, a Legendary Weapon. The skin, however, shouldn’t be anything too fancy. If people want a fancy skin Legendary, they should chase those like anybody else. A sword/shield/bow/staff/ect with the flexible stats of a Legendary, however, would be a good reward for doing all the upgrades.