Wisdom teeth, also known as third molars, grow at the back of the mouth, which is the most difficult area to maintain oral hygiene. It's hard to brush and floss wisdom teeth. They may be impacted, decayed, cause pain and gum infection. Extraction is the most common treatment option if they cause any trouble.
In this case, the decay on the wisdom tooth just touched the nerve. From the 2D x-ray, it seemed a relatively simple extraction, but it's actually quite difficult because of the hook-like root. The tooth was removed successfully without breaking the root and the recovery was uneventful.