Arcana items are among the rarest items in Dota 2. They come with extensive customizations such as particle effects, animations, icons, sounds, and more as it cannot be deleted in the Armory.
Lina was the the first hero that received an Arcana, as the "Fiery Soul of the Slayer" arcana item was based on Lina's alpha character design.
Legion Commander's "Blades of Voth Domosh" arcana bundle saw the hero wielding dual blades as well as customized effects for her abilities.
Terrorblade's "Fractal Horns of Inner Abysm" arcana is the most interesting one out of all arcanas, as the abilities' particle effects can be changed by inserting a different type of Prismatic Gem.
Following his explosive debut at TI4, Techies' "Swine of the Sunken Galley Bundle" is currently the cheapest Arcana available on the Steam Market.
Shadow Fiend's "Demon Eater" Arcana item was released in 2014 after TI4, as the item was commonly used by mid laners and currently carry players.
Phantom Assassin's "Manifold Paradox" Arcana had an interesting backstory, as the blades were given by the Oracle in the short comic, The Contract.