Legendary Weapons
To unlock vol. ll you need to craft whatever the experimental version of the legend is. Should be found under “Legendary compontents” by the artificer. That should award you the collection Bifrost ll.
Optimise [OP]
First, Bifrost is a pre-HoT legendary. You’ll be after items/challenges in core Tyria to craft it.
Use gw2efficiency.com to track the materials you’ll need. Don’t be surprised! The precursor and the legendary that follows require quite a lot of materials and gold. And all you really get is a skin, so you’d better love it! If stats are what you want, it’s far cheaper to craft ascended.
The wiki should give you most of the information you need to find the various collection items.