The Long Hunt: Book 4 of The Strongbow Saga;

The victorious Viking army has returned to Denmark from Frankia, after capturing Paris and extracting an enormous ransom from the Franks in exchange for leaving their kingdom in peace. Now Halfdan, with the aid of Jarl Hastein and his warriors, is ready to bring Toke, the chieftain who murdered Halfdan’s brother Harald, to justice. The Strongbow Saga is an epic tale of one man’s unstoppable quest for justice and vengeance that carries him across the 9th century world of the Vikings. In The Long Hunt, book four of the Saga, Halfdan sees his quest for vengeance turn into a long and grueling pursuit, as Toke flees eastward through Danish waters and across the Baltic Sea. But though he is now outnumbered and on the run, Toke proves to be a cunning and ruthless foe, and his pursuers will soon find themselves to be the ones in danger.