Pint-Sized Slasher mask

The Pint-Sized Slasher Gear, is obtained during the tranquility lane quest, if you decide to help Betty to kill all residents of Tranquility Lane. The gear consists of a knife and a clown mask. They are located in a Doghouse, and are automatically equipped when you receive them.

The knife that you receive for this portion of the quest has a lot more power than a normal knife, and higher critcal chance. However, in Tranquility Lane your Pip-Boy 3000 is replaced with a wristwatch, and there is no way to check the knife's power.

At the end of the quest, these items are removed from your inventory automatically.

Trivia
One of the newspapers shown when loading saved games mentions 'The Pint-Sized Slasher' in a side story.