"Die" is a common singular form of "dice". The relative prevalence of "die" vs "dice" for this is regionally dependent. In general, English can't make up its mind with a lot of things involving words which end in a "sss" sound.

Die is singular, dice is plural. 1 die, 2 dice, 3 dice, …

It used to be something people were corrected on, but these days dice is accepted as the singular too and die is starting to be seen as a little antiquated.