Perhaps a more fitting name for the piece would be the Vortex. Or the Alternate Dimension Portal.
Invent the new worst/most useless chess piece ever!

The double-agent. One of the pawns is replaced with a double agent randomly. Upon reaching the other end of the board you check underneath the pawn to see if it has the doubleagent sticker (or if playing online a computer just tells you). If it's the double agent, the pawn becomes your opponents and marches against you! the effect is only counted once, if your opponent gets it to the other end of the board it can queen like a normal pawn

The incessant whiner: it instantly turns a "winning" position into a draw and then comes into the forums to complain about the unfairness of the stalemate rule or unsportsmanlike nature of perpetual check.

pawns are numbered according to files ie. A pawn = #1 and after moving it two times it disappears off the board
the B pawn = #2 disappears after moving it three times
c pawn = #3 disappears after moving it four times
d pawn = #4 disappears after moving it 3 times
e pawn = #5 disappears after moving it 2 times
it would certainly make you think twice about moving some pawns now wouldn't it? and definitely redenders itself useless after a few moves.
interesting comments on this thread op, good work

The tsunami: it's a glass full of water sitting next to the board. If a player is in check, he may try to avoid the check by throwing the water at the enemy pieces. May also be used as distraction by throwing it on the opponent's face.

The Crow, uses a magnetic levitation to float above the chess pieces. Its function is to scout out enemy position.

A dice. Each move both players have to say a number. The numbers must be different. The dice is rolled until one of the two numbers that was mentioned appears. The player that guessed the number gets the move.

How about the phenolomorphin? Its completely invisible so there is no need to spend money to have it made up. Each player gets one and each player can place his on any square of the board that he chooses. Now the thing to remember is that both players can place any of their pieces on the same square that this piece is already occupying and the phenolomorphin does not affect any of the pieces in any way. As an interesting experiment I suggest that you ask your opponent if he wants to play with this new piece and see what he says.

That could be useful in the opponents half of the board, it would probably encourage more attacking play and get more children playing.

The Peo! Derived from the Spanish peón, in turn derived from the French peon, the piece has been demoted and lost its n. Unfortunately, its name now means flatulance in Spanish. It summons a noxious cloud of methane, phosphorous, and sulphur to annoy the heck out of the present pieces, mainly the King's feeble-minded cousin Joey!
the troll. whenever someone has a winning position, it trolls you and gives the other player a mate in 1
I don't think trolls have the ability to do that. And I think it's reasonable to say that I'm, well, more or less an expert on what trolls can and can't do.