The Diggers, appeared in the episode "The Tale of the Virtual Pets".
Not much is known about their past, it's unknown who created them and for what purpose. They were beings of sentient artificial intelligence.
They were highly intelligent, planning to take over the world, capturing all humans and replicating themselves as each one they captured.
When a victim was captured they were dematerialized and placed into a handheld toy like device. Eventually they would plan to connect the handheld to transferenece unit. This would upload the captured human's knowledge. to their main computer program.
Meanwhile a Digger would replicate itself as the human it captured and be identified by having a small purple bulldog tattoo on the back of their neck.
A boy named Tom discovered their website, by slim chance. He was the first victim, the Digger impersonating him passed out free toy handhelds saying they were Toy Virtual pets called Diggers. Tom's friend Isabel and two other kids got captured and replicated as well. Also a girl named Kate was offered a virtual pet toy, but not being interested in electronics she refused it.
Soon, they connected the handheld devices to the transference unit to upload their minds into their mind and possibly their whole bodies into their main computer. Kate arrived just in the nick of time. First Kate tried to unplug the computer, by they didn't let her. Next she was instructed to shut it down, but that didn't work either.
Tom instructed her to locate the Digger application and delete it, but that also didn't work. Then Kate was instructed to run all the computer programs at once to overload and crash the computer, this also didn't work.
Then just as the upload was beginning, Kate used a mirror to reflect the energy beams back into Tom's computer causing it to literally explode and meltdown.
This seems to have destroyed their program, but one Digger nearly got away. The digger stored itself in the last remaning handheld device, but luckily Kate stomped on it and destroyed it.