Tie-Ins: Highlander

Highlander is a series about immortal sword guys cutting each other's heads off.  With hundreds of years of backstory, the series is ideal for spin offs.  The film series is notoriously horrible, there are some animated versions, with a better reputation, and in the middle there is the TV series, which had it's own novel and audio play spin offs.

The only novelization is for the first film:

• 1986 Highlander by Gary Douglas aka Gary Kilworth

Continuing adventures from the TV series:

• 1995 The Element of Fire by Jason Henderson
• 1996 Scimitar by Ashley McConnell
• 1996 Scotland the Brave by Jennifer Roberson
• 1997 Measure of a Man by Nancy Holder
• 1997 The Path by Rebecca Neason
• 1997 Zealot by Donna Lettow
• 1998 Shadow of Obsession by Rebecca Neason
• 1998 The Captive Soul by Josepha Sherman
• 1999 White Silence by Ginjer Buchanan
• 2000 September Highlander: An Evening at Joe's  - anthology written by the crew of the series

There's an authorized fan series Imagine: For Love's Sake, Beyond Infinity, and Code Name Immortal.