Dice Probability Dungeons and Dragons

The answer is $frac 1 {20} cdot left(frac 1 {10}right)^4 = frac 1 {200000} $.

However, since you mentioned D&D....

Let me tell you that information theory also tells us that the more unexpected something is, the more knowledge you get for receiving it.

Dice rolls are a type of divination that go beyond the pure realm of math-based probabilities. I've estimated that a dice roll, with flat faces, on a hard surface will give you about 130 bits of information on each roll. This huge number is enough to get data from almost every atom of the universe (science estimates this to be not much more than 10⁸²).

Rare events happen all the time. If you imagine all the games of D&D ever played and think of the dice roll sequences that you would want to ask here about you can see that some of them would happen sometimes.

Your confusion comes from asking after you know it happened to you.

Winning the lottery is a very rare event. But someone wins. They think they are special (well, they are lucky).

More discussion here:

Probability of two people from two different countries meeting in a different country and meeting each other

It's simply $frac{1}{20}cdot (frac{1}{10})^4$, or 5 in a million.