Which practice is sometimes characterized by extreme nationalism?

It's Fascism

It influenced by national syndicalism, Fascism originated during the Reign of Benito Mussolini of Italy. He somehow managed to convinced his people that Italy is a Superior Race and the rightful heir if this world. So he wanted to restore Italy's Glory back to the Roman Empire's era.

A thing which happens when we have extreme nationalism would be that far right groups get established which promote far-right clubs, organizations, events, and similar things. These then serve as the basis for the recruitment of more far right people who are extremely nationalistic. All of this can eventually lead to xenophobia.