Three vampires

Three vampire hunt brothers decide to hold a competition to see which one of them is the most powerful

The first brother is the strongest.
"Watch this," he says, and takes off at nearly 100 miles per hour. Two minutes later, he returns, his mouth covered in blood.
"What happened?!" his brothers exclaimed.
"You see that mansion over there?"
"Yeah?"
"Well, I went over there and sucked each and every last family member dry. They are all dead."
"Wow!" his brothers say. "As expected, for you are the strongest."
The second brother to go is the oldest.
"Watch and learn, boys," he says, and takes off even quicker, at 150 miles per hour.
Five minutes later, he returns, both his mouth and his neck covered in blood.
"What happened?!" His brothers exclaimed.
"You see that village over there?"
"Yeah?" They said.
"Well I went over there and killed every last person in the entire village. There is not one left
alive."
"Wow!" his brothers say in awe. "As expected, for you are the oldest and have the most
experience."
The third brother is the fastest. Not to be outdone, he says "Watch this, and don't blink or you might miss it."
He flies off, faster than the rest of them, going at least 200mph.
In only ten seconds, he returns. His entire mouth, nose, and neck are covered in so much blood, it stains the front of his shirt.
"What happened?!" his brothers exclaimed.
"You see that giant tree over there?"
"...Yeah?"
"Well I sure %@&+ing didn't."

The punch line of this joke is funny because the third vampire reveals in a sudden manner that he ran into the tree. This is slapstick comedy.

It is also funny because the setup of the joke follows the conventions of a fairy tale. The setup makes it seem like the third brother will do better than the first two, which is traditional in a story with three brothers who go out one after the other.

Finally, the manner that the third vampire speaks is not as high class as I would expect a vampire to speak, so that also contributes to the humor. Having a vampire swear is pretty surprising.