This is an undocumented secret hidden in DUSK's E2M1 level. Here's how you can get the hidden sword, and get back into the map once you are out of bounds!
Other DUSK Guides:
- Cheat Codes.
- All Secrets Guide (Episode #1).
- The Foundry Secret Level (Episode #2).
- How to Find the Secret Achievement Duskbaby.
- How to Find the Secret Achievement Go Away.
First you'll want to grab a Climbing Thing. You can find one on top of the tower in back area of the map, or you can use the one found in the buried secret area. The video demonstration above uses the latter.
Once you have one, you'll want to quickly bhop over to the back-left corner of the map, where you can find a secret teleporter. Use the teleporter inside of that room to get to the secret area.
When you get to the secret area, MAKE SURE YOU GRAB THE CROSSBOW, because you will need it to get back into the map!
Once you have it, use the Climbing Thing to climb over the wall of trees near the campfire.
You should end up in the outside area of the map!
Walk around the inner walls of the level until you're outside of the area where the level starts.
Follow the dirt path, and you should find the hidden sword, as well as a "Go Away" message from the developers, surrounded by a bunch of blue keys.
(Oddly enough, this message currently doesn't count for the Go Away achievement)
Now that you have the sword, you'll want to head back to the area where you jumped over the trees, near these two large rocks.
You will need to climb on top of the larger rock, jump forward while using the crossbow, and continue firing arrows downward so that the knockback lets you get back into the secret area.
If all goes well, you should be able to use the teleporter to get back into the map, and now you can use the sword for the rest of the episode!