The fourth layer of Acheron is lightless but filled with fast-flying, razor-thin shards. Some shards are little more than needles, while others are miles wide. The largest shards have their own objective gravity, like the cubes of the upper layers, as well as a breathable, if icy cold atmosphere. The constant blizzard of blade-like shards makes Ocanthus inimical to creatures and objects alike.
The shards are black ice, frozen into thin layers. Their collisions break them into progressively smaller shards, and eventually into needles and then dust. The shards all originate from a single source: the night-black boundary of Ocanthus, a sheet of infinite, magically charged black ice.
No one knows whether the ice sheet is a boundary or a barrier between Ocanthus and some deeper, more sinister layer. Some say that the ice is the source or destination of the River Styx, and that every memory stolen by the river still exists, frozen into the black ice. Whatever the truth, the ice sheet has gravity, and it is possible that it is simply a shard of black ice so large that it is mistaken for Ocanthus' boundary.