When crafting heirlooms in Dungeonborn, there are two legendary items that you can craft - The Divine Sword and The Divine Crown. The way to get these is well-hidden, and will take a few steps before you can unlock both items.
Note: in order to get these items, you'll need to venture into Sinner's End, which requires you to have at-minimum full greens to compete with other players.

How to get the Moon Pendant in Dungeonborne
In order to get the unlock the Moon Pendant Heirloom, you'll need to get your hands on the Moon Pendant Fragment, an Epic Material that you can only find at one place.
Once you're at your Camp in the Main Menu it needs to be:
- Nighttime
- Full Moon
If both of those conditions are met - go to the Training Room under the Selection Menu. Once you've loaded in, head inside and straight towards the altar you see inside. Pray at the Altar, shown below, and you'll get the Moon Pendant Fragment in your inventory.
Open the menu and return to Lobby, and you'll then be able to craft this Legendary Necklace for 1 Mithril Token or 2 Star Fragment.

How to get the Divine Crown in Dungeonborne
In order to Unlock the Divine Crown Heirloom, you'll need to head to Sinner's End (Casual or Classic) and pray at a specific shrine.
The best place to find the Divine Crown Shrine is on the north, and middle, part of the Sinner's End map. You'll see an open hand, located at the very top of the map, that doesn't have anything but a small glow in it, and the have people at the tips.
Pray at that shrine, to collect the Divine Crown Fragment. Now, you'll have to escape the map via Portal to keep it - once you've successfully done that, you'll be able to unlock, and craft this Legendary Helm for 1 Mithril Token or 2 Star Fragment.

How to get the Divine Sword in Dungeonborne
Once you've unlocked both the Moon Pendant Necklace, as well as The Divine Crown - you'll be ready to get the Divine Sword Fragment!
With the Divine Crown (and/or Fragment) and the Moon Stone Necklace (and/or Fragment) in your inventory OR equipped, drop into Sinner's End. You'll find the Floral Sarcophagus at the bottom, and middle, of the map as highlighted in the map below.

How to Open the Floral Sarcophagus: With the the Divine Crown and Moonstone Necklace (or fragments) in your inventory simply interact with the Sarcophagus, and it will use up both - offering you The Divine Sword Fragment in their stead, and completing the Sealed in Blooms Achievement.
Now, you'll need to extract so that you can unlock this Legendary Sword Heirloom, which can be unlocked and crafted once you've returned safely to camp for 1 Mithril Token or 2 Star Fragment.