She 'went into mourning' for her dead husband. [=she began to mourn for her dead husband]
She is still 'mourning' her husband, who died last year.
She is still 'in mourning' for her dead husband.
(..) by the blood of Odin... to avenge my father's death. Why do we wait? We must mourn Ragnar. It's revenge Ragnar wants, not mourning. Every Viking wants to revenge the death of Ragnar. Then let's go to England now... for the girl and the slave. We don't even know (..)1958 The Vikings