Unlike popular belief, Samthainn(pronounced sow-en) is not a god of death. He is a god of smiths, brother to Cian and Goibniu.
Cian had a cow called the Glas Glasheen, everyone conveted it so it had to be watched day and night, however, Cian needed to go into the forge so Mac Samthainn guarded the cow, however, Balor the giant disguised himself as a red-headed boy and told Mac Samthainn that Cian and Goibniu were planning on using his steel and make his sword out of iron. So, Samthainn handed him the reigns and confronted his brothers.
Cian was furious, and after consulting the druidess Birog, he seduced Ethne, daughter of Balor, which resulted in the birth of Lugh.