What the Dark Shell does.

the dark shell goes to the slowest character in-game, and explodes like a blue shell but its purple. And to unlock it, type darkshell when making a new item

Wait maybe it should go to the last player, or hit the fastest character?

Kind of correct. The dark shell only targets the player that is in last place, regardless of speed.

ok then

So does this mean that if everyone in the lobby has the same stats does the dark shell targets a random character?

No it always targets last place its not based on stats or anything