I have been pretty good at avoiding online guides for MoM so I had only hazy notions about legendary weapons when Throg was offered his first one at the end of volume 2 chapter 1. To be honest I found it a bit confusing at first. None of the weapons on offer looked particularly impressive but I had to choose one so, Throg being a Dwarf, I picked the axe and then brought it to the dwarven forge master for identification. The related quests left me with a basket of upgrade bits (called relics) so I duly set about populating three upgrade slots (setting, rune and gem) with the best relics I had. A scroll alllowed me to name the weapon and change the damage type.
So far so good but it was at this point I spotted that my legendary weapon also comes with inherent bonuses (called Legacies). Sadly this axe has a useless set of legacies including several bonuses to the virtually unused "Hedge" skill and a bonus to the least popular "Ardour" stance. I regret wasting my best relics on the weapon but no doubt Throg will stumble across others in his continuing adventures.
At first this purple quality legendary axe was no better than a standard purple quality axe but the next quest sent me out to fight with weapon so that it could gain experience and level up. It seems that levelling up the axe earns legacy points which can be used to increase the damage or enhance the legacy buffs. An hour or so of questing in Eregion brought me up a quarter level to finally hit 51 and my axe up six levels to level 7. It seems that weapons gain levels a lot quicker than players (at least at the beginning) which is probably no bad thing.