The odds of rolling any single number on a single roll of an 8-sided die is 1/8

The odds of rolling any combination of specific numbers for multiple rolls is 1/8 * (1/8 for each roll).

So the odds of rolling a 1 followed by a 5 based on two rolls is ... 1/8 * 1/8 = 1/64

If you want the odds of rolling a 1 four times in a row ... It would be 1/8 * 1/8 * 1/8 * 1/8 = 1/4096

Of course, this is assuming you don't NEED the die to roll a specific number in a D&D situation. If you do, the die takes on a mind of its own to defeat your chances. LOL