The Obsidian Fence is a construction element that can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that can still be found on older worlds), but only after the crafting-recipe has been unlocked by
- crafting (or taking) Stone Fences which are unlocked after crafting Wood Fences
- creating Obsidian Slabs from Obsidian bars in a Processor
- crafting Obsidian Brick Walls from Obsidian Slabs, Stone Rods and Globs of Goo
To craft 8 (pieces of) Obsidian Fences at a time you'll need (as of R30 in April 2016):
- 1x Melted Wax made of Beeswax in a Forge or looted from Wood Treasure Chests or Stone Treasure Chests
- 1 Stone Rod made of Stone, Limestone or Bedrock in a Processor
- 2 Obsidian Rods made of Obsidian (Bars) in a Processor
- 2 Obsidian Slabs made of Obsidian (Bars) in a Processor
However breaches in the enclosure or defence can be created by placing blocks or liquids next to fences that would create "steps" or by placing items (like torches) directly on the top of Obsidian Fences.
Similar to Iron Bars, when only one Obsidian Fence is placed on the ground, it will look like a simple short post, and if you stack these, you can create long poles or lamp posts (by adding lamps of any kind on top).
When blocks or solid items like gates are (placed) right next to an Obsidian Fence, the fence will connect to that by enlargening horizontally. Placing two or more Obsidian Fences next to each other will automatically cause them to connect, also crosswise, but not diagonally (not crossing over more blocks than where they have been placed). Obsidian Fences can fill holes in the same way. Gates will not extend themselves to link with Obsidian Fences, only the other way round.
Since R44 (June 2017), fences can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.