Laser Sword
From HelpMeWithMyGameProblems.com
The Laser Sword is the reward for surviving 5 days in Infinity Mode. Unlike the unlockables in 72 hour mode, the Laser Sword appears in the Security Room the first time you enter the room after having been awarded the achievement.
The Laser Sword operates like a Katana, except that it lasts for 300 kills. The Small Chainsaw is arguably a better weapon, due to the fact that its durability can be increased by a factor of 27 (if the correct three books are held while it's used), whereas there are no books that affect the Laser Sword.
