He is perhaps best know for outlawing the old Nordic pantheon of deities, and for being one of Marukh's chief supporters.  He is also believed to have been the last monarch to wear the legendary Jagged Crown. 
Borgas was killed during the Wild Hunt of 1E 369 as he traveled to Cyrodiil to urge a joint war against the Bosmer.  His body was secretly returned to Skyrim and buried with the crown in the Nordic tomb of Korvanjund. 
Following his death, a Moot was held to determine his successor. Supposedly, Jarl Hanse was the obvious choice for succession, however the Moot's failure to appoint him High King ultimately led to the War of Succession. 
In 4E 201, Galmar Stone-Fist learned of his place of burial while on a search for the Jagged Crown. The Dragonborn was sent to retrieve the crown from Korvanjund where a skirmish between Imperial and Stormcloak soldiers took place. Deep within the ruin old king Borgas was found slumped on a throne wearing the crown - his ancient corpse having taken the form of a powerful Draugr.