not really a weapon but you can get it by using special weapons a lot
kinda like a religion thing. theres a good and evil side in a human in religion saying. I think christianity has that or maybe some other religion has that concept also.
the point is: using special weapon can taint a player's soul into darkness. so when a certain point the soul has been tainted. he/she will become dark and gain lots of power, but at a very terrible price.
dark souls abilities: guns will get power bouns (gun damage times 1.5)
immortal for a short period of time (about 1 minute)
knife will do double damage
consequences: start with only half HP (cannot buy kevler)
lost control of your player for 25 seconds
stop gaining credits
your player will be clad in complete black
regular weapon cost will double
cause the player to shake violently, makes hard to aim
every player start with 100% pure goodness. when they start using special weapons the % will go down depend of how powerful the special weapon is. once at about 90% of the goodness is lost. he/she will gain the darksoul. to return back to light you need to play without using special weapons. 1% gain=2 hours of play.once you gain back to 50% you will rid your dark soul.