Lucius Artorius Castus's cause of death is unknown, whereas King Arthur died during the Battle of Camlann.
Comparison of Lucius Artorius Castus and King Arthur Death Unknown date and circumstances; probably died at an advanced age, potentially during his procuratorship of Liburnia (where he was buried). In Welsh literature, traditionally stated to have died during the Battle of Camlann (of unknown location in Britain); his burial site was unknown to medieval Welsh. 
