What is the most serious type of brain injury?
A blood clot in the brain.
In other words, a stroke. A stroke is when you have a blood clot in the brain, or when a vein in the brain bursts. Usually, it's fatal, but I know a couple survivors. If it doesn't kill you, it will change you, drastically. For example, your vision is impaired, you walk worse, you talk worse, etc.