From Warhammer 40k - Lexicanum
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Like all the Primarchs, Roboute Guilliman was separated from the Emperor and came to rest on a distant planet, in this case, Macragge. Roboute's arrival on Macragge was a portentous time, and many reported strange sights. Most notably, Konor, the Battle King of Macragge, dreamed of the [[Emperor]], and found himself beside Hera's Falls in the Valley of Laponis. Upon awakening, Konor assembled his bodyguard and rode to Hera's Falls. When they finally arrived, they found a small child there. Konor took him in, naming him Roboute, which means "Great One".