The Spiny Shell (also known as Blue Shell or Spiny Blue Shell) is an item in Super Smash Bros. Clash. It's a homing item that, when picked up, would fly across the field and tracks down the player who is in the lead and blow them up, similarly to how it works in the various Mario Kart titles.
The Spiny Shell's design is based off it's appearance in Mario Kart Wii.
When a Spiny Shell is picked up, it flies into the sky and tracks down the leading player. After about ten seconds, the Spiny Shell homes in onto the leader and crashes onto them, causing a huge explosion that can hit other players. The explosion is unblockable and has the Dodge Break property for whoever it homed in on, but there are ways to dodge it. Characters like Ryu can use moves with invincibility frames, like the Shoryuken, to dodge the explosion. Some characters can also run away from it using a Bunny Hood, as the Blue Shell does have a speed limit. Characters other than the one the Spiny Shell homed in on can block it, but can not dodge it.
The Spiny Shell KOs Mario at about 25% and deals 60%. Against players who were not homed in on and block the explosion, they take 10% chip.
The Spiny Shell will only show up in matches featuring 4 or more players, and can not show up at all in team battles.
The Spiny Shell first debuted as it is in Mario Kart 64, although this version of the item did not have wings. It would still home in on the first place player, but an explosion would not be caused and other players could be hit by the shell on it's way to destruction. It stayed like this in Mario Kart: Super Circuit, but in Mario Kart: Double Dash!! the Spiny Shell gained wings, so it could only damage the first place player, and caused a huge explosion that sent the victim into the sky on hit. The explosion could hit other players though. It's traditionally a fairly rare item obtained for lower place players.
It's also pretty infamous, being the cause of some rage across the entire Mario Kart community. Magmortar75 himself admits he hates the item. The rage is most apparent in Mario Kart DS and Mario Kart Wii.