The Halloween Weapons, or Horror Weapons, are limited availability reskins of the tier 11 weapons themed on Halloween.
The Skull-splitter Sword was first released as a standalone item at Halloween 2013 in Release 17.3, dropping in various locations as a soulbound reskin of the T11 Skysplitter Sword. This in fact was the first reskinned item in the game.
Halloween versions of the other T11s were launched the following year in Release 24.1.0 (Oct 2014), available as alternate loot drops from dungeon bosses and quests, and from the Zombie Horde. The items have since made regular appearances at Halloween and in event chests.
These items are not currently soulbound and can be traded.
They have previously been soulbound at various times.