Aasimars in Hallo
Aasimars are the result of a celestial being coupling with a mortal. In general Aasimars have lighter and fairer coloring of their mortal parent’s race and lean towards a more goodly disposition.
Before The Abandoning celestial beings were more prevalent on the mortal plane. However, Aasimars are still an uncommon race and rarely seen. Of those that exist, many do not even know they are related to a celestial being.
See page 286 of the Dungeon Master’s Guide.