if you are going to reference Slaine stick to the seminal works by Simon Bisley Clint Langley is very good but in the case of Slaine he is simply walking in Bisleys shoes, look at The Horned God 1-3 graphic novels. Also Slaine was a Celtic hero rather than Viking. The character was an amalgamation of saga figures such as Cuchullain, in the Celtic sagas the Formorians are closest to the Vikings in terms of concept although they predate Viking expansion by about 1000 years. The Celtic sagas took place during the Iron Age (the Celts expanded into the British Isles about 500BC) and the Vikings reached British shores in late 700-800 AD.
Thanks Simon I'll definitely look up "The horned God".(Clearly there was a lot of background information that I didn't realise)