Sovereignty goddess is a scholarly term, almost exclusively used in Celtic Studies (though parallels for the idea have been claimed in other traditions, usually under the label hieros gamos). The term denotes a goddess who, personifying a territory, confers sovereignty upon a king by marrying or having sex with him. Some narratives of this type correspond to folk-tale motif D732, the Loathly Lady, in Aarne and Thompson's Motif-Index. This trope has been identified as 'one of the most well-known and often studied thematic elements of Celtic myth'. It has also, however, been criticised in recent research for leading to 'an attempt to prove that every strong female character in medieval Welsh and Irish tales is a souvenir of a Celtic sovereignty goddess'.
There is some evidence in Greek and Roman accounts of historical Celtic women that leading women such as Camma and Cartimandua might in antiquity actually have been associated with goddesses. It is also clear that medieval Irish rituals inaugurating a new king sometimes took the form of a banais ríghe ('wedding-feast of kingship'), because the king was imagined symbolically to be marrying his dominion, and that similar rituals known by the term feis might involve both sexual activity, and horses (in turn evoking the idea, prominent in modern scholarship, of Celtic horse-goddesses). Most luridly, Giraldus Cambrensis, in his 1188 Topographia Hibernica, claimed that at the inauguration of the king of the Cenél Conaill, the successor to the kingship publicly sexually embraced a white mare. This would then be slaughtered and cooked into a broth in which the king bathed, before he and his people drank it.
However, the type-text for the idea of the sovereignty goddess is the medieval Irish Echtra Mac nEchach ('the adventures of the sons of Eochaid'), in which a hideously ugly woman offers the young men water in return for a kiss. Only Niall kisses her with conviction, and moreover has sex with her, whereupon the woman becomes beautiful and utters the verse