The Armanen row is Guido von List's, published in 1908, and each of its eighteen characters sits on a historical original that he renamed and reinterpreted.
The character behind it
Hagal corresponds to the Younger Futhark character whose Old Norse name was hagall, meaning hail. The rune poems gloss it as the coldest of grains, a hard seed that falls and melts. The character is well attested in the Viking-age inscriptional record doing ordinary work as the letter H.
What von List did with it
He kept the H value and the name in Germanised form, and then went considerably further with this character than with any other in the row, treating it as the rune from which the rest can be generated. That reading, and the drawn form he associates with it, are his own and have no counterpart in the historical material.
What is his and what is not
The hail-grain gloss is attested in the medieval rune poems. The idea of a single generative mother rune is not. Keeping those apart is the whole discipline required when reading this entry, because von List presented his construction as a recovery, and runologists do not accept that account.
Reception
The Armanen row was adopted in volkisch circles and by National Socialist organisations during the 1930s and 1940s, and characters from it appear in the insignia of those organisations. The atlas states this rather than omitting it.