From Lexicanum

The Heliopolis was a central assembly hall on the Imperial battleship Pride of the Emperor and was used by the Emperor's Children Adeptus Astartes Legion in the years of the Great Crusade.

The door to the Heliopolis was the Phoenix Gate which waited the entry of the Primarch of the Legion, Fulgrim. Once inside and when everyone took their seats, Fulgrim typically gave his speeches and on the days before a battle, a feast as well as celebration was held within the Heliopolis. ¹

Source

• ¹:Fulgrim by Graham McNeill