It gives you around 3-4% of your experience.
The title of this Garrison Mission is "There can be Only One." This title also shares those words as a phrase from the movie series "Highlander" and it's spin off television show "Highlander the Series". Quoting from the Highlander Wiki: "It implies that all Immortals must fight and kill one another until only one remains standing; this "One" shall receive The Prize."
Must equip your follower with a very very good sword.
Reference to a commonplace quote from character Duncan McCloud in the TV series The Highlander
Garrison Timer Mobile App
[Upon completion your 随从 will gain: ]
Missions and Followers math for maximising success chance
Warlords of Draenor Garrison Missions Guide