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Dark Souls: Prepare to Die is an expanded version of the console game Dark Souls. Next to the original gameplay it adds in additional content slip-streamed into the main game:
General gameplay tweaks
Multiplayer arena as a training ground for online PvP battles: duels (one versus one), team battles (two versus two) and four-player FFA matches
New bosses (Black Dragon, Sanctuary Guardian, Artorias of the Abyss)
New enemies (Wooden scarecrows, Chained Prisoner, Stone Knight)
New NPCs (Hawkeye Gough, Elizabeth)
New weapons and armor
Oolacile as a new area, split up into four smaller sections
For the PC platform this is the first release of the game.

Artorias of the Abyss is an expansion pack for Dark Souls, first made available as included in the PC version, Dark Souls: Prepare To Die Edition, on August 24. This content adds a sideplot to the original Dark Souls story that sends the Chosen Undead back in time to Oolacile, chronicling the legend of the Abysswalker, Knight Artorias. New Locations, Enemies and Items are available, as well as an Arena Mode for more competitive Player versus Player online play. This expansion also serves as a new Patch, changing or fixing many elements of the console versions of Dark Souls.
See also: PTDE content and changes, concerning strictly the PC version.