ISIS (IS, ISIL, Daesh) are the worlds best funded terrorist group.
They follow an extreme interpretation of Islam with their end goal being the creation of an Islamic State.
They are notable for their branding, often wearing the same uniforms and producing slick propaganda videos.
Danger to the UK mainland
Dying at the hands of ISIS is joint 6th (along with sinkholes) as the most likely threat to someone in the UK with 0 deaths per year. This is after air pollution (40,000 deaths/year), household accidents (6000 deaths/year), processed meat (3300 deaths/year), crime (600 deaths/year), bathtubs (8 deaths/year).