Kurtuk is a sprit that can be found in the darker parts of the spirit world, it feeds on dark energy. (Much like the energy Unavaatu is able to bend)

It was first discovered by Korra and Asami when they went on "vacation" to the spirit world. Korra asked if Asami wanted to go check out the darker parts of the spirit world, when they got there Kurtuk was very angry due to the fact that he had lost one of his toes to Kuu. The toe stealer...

Korra defeated Kuu and took Kurtuks toe back. After that day Kurtuk felt the need to help others like him. (Who had lost their toes)

So if you ever lose a toe to a dark spirit make sure to call Kurtuk and he will make sure everything is going to get back to as they should be. (All of your toes on your foot)