Web

Web is listed on the wiki as a Bingo Challenge reward, not a box drop. During the 2015 Halloween Event, which ran from October 29 to November 4, 2015, the only way to earn it was the Bingo Challenge: a player had to finish all of the hard-difficulty challenges. When the event ended that route closed, and now the Legendary knife changes hands only through trading.

The design is dark and simple. The blade is solid black with red webs imprinted on it, the edge is stone gray, the guard is red and the handle is solid black.

Part of the Web Set

Web has a partner gun called Rupture, and the pair makes up the Web Set. The wiki lists no box and no price for it, because it was a reward for completing challenges rather than something you bought. That makes it a clear example of an event item whose whole story is tied to a single week of 2015.