Permanent Shadow Form (often abbreviated as Perma-Shadow, Perma SF or Perma) refers to any build that focuses on indefinitely maintaining Shadow Form in order to remain invincible against spells and attacks. Such builds are often used to solo farm difficult areas like the Underworld or the Fissure of Woe and to solo boss farm in Hard Mode.
As a result of the May 22nd, 2008 update, Shadow Form received a considerable buff. Its duration was increased and it can now last over 25 seconds with a reasonable investment in Shadow Arts and an enchanting mod. The new Shadow Form is also what makes Assassins a better runner than primary Dervishes most of the time.
Shadow Form can be maintained indefinitely by combining Deadly Paradox with either Glyph of Swiftness or Arcane Echo (although the latter version requires precise timing). The Assassin/Elementalist combination is particularly powerful (as compared to the Assassin/Mesmer combination) because it has access to a wide variety of powerful Area of Effect spells. Because maintaining Shadow Form is very energy intensive, it is not uncommon for builds to use Channeling, Glyph of Lesser Energy, and/or Spirit of Failure to maintain a high energy pool.
Keep in mind that Shadow Form will not make a player 100% invincible, as Shadow Form does not protect him or her from Attacks, non-Spell skills, non-targeting Area of Effect, or skills that indirectly affect the player. For example, traps created by Obsidian Behemoths in the Underworld will mostly kill a Permanent Shadow Form Farmer if he or she isn't careful.
The possibility of indefinitely maintaining Shadow Form has caused a lot of controversy within the community. Some players believe that Shadow Form allows players to farm in various difficult areas without effort (particularly in Hard Mode and in the Underworld) and many players consider such builds to be a gimmick. ArenaNet has officially responded to this issue in their June 18th Developer's update stating that they will "evaluate the high efficiency of Shadow Form farming builds" and will "monitor this issue and may make adjustments in the near future." As such, the fate of this build may appear to be very grim, since ArenaNet had made hostile changes towards farming in their previous updates. The July 2nd update added a 50% damage penalty to Shadow Form, thus making permanent shadow form farming much less effective, and time consuming. A later update on August 7th 2008 slightly rebuffed Shadow Form to 33% damage penalty to allow some players to effectively farm bosses while the Mindblade Spectres in the Underworld undergo additional buffs to balance it.
Ever since, ArenaNet staff were aware of the new trend of the permanent Shadow Form and made sequential changes with Shadow Form and do not want to rule this form of build out as an option. In the December 11th, 2008 update, the duration and recharge were reduced. Although it is still maintainable for assassin primaries only, the purpose of the nerf, according to Developer Updates, were to "force players to cast Shadow Form more frequently" and to increase "Energy pressure." In other words, while the build is still effective, it is no longer energy efficient as it was before. Even with using a high energy set, Glyph of Lesser Energy or Spirit of Failure, many players have reported energy problems when trying to maintain the build and at the same time killing the enemy.
The February 25th, 2010 update completely changed the functionality of Shadow Form where it no longer completely protects the player from physical attacks while limiting the amount of damage Shadow Form can inflict to curb tanking and farming purposes, thus making perma Shadow Form builds significantly less useful for most farming purposes.