There are several bands using this name:
1) Relentless is a punk rock band from Joensuu, Finland.
2) Relentless is a hardcore band from Sydney.
3) Relentless is a death metal/grindcore band from Sweden.
4) Relentless is a classic hardcore band from Long Island.
5) Relentless is a house band from San Francisco, California.
6) Relentless is an avant-garde saxophone duo.
7) Relentless is a hip hop duo from Brisbane, Australia.
8) Relentless is a hardcore band from Twin Cities, Minnesota.

Read more about Relentless on Last.fm.