Heimskringla Saved Head

Óðinn went with an army against the Vanir, but they put up a good fight and
defended their land, and victory went alternately to both sides. They each
raided the other’s land and did damage. But when both sides grew weary
of this, they arranged a meeting of reconciliation between them and made
peace and gave each other hostages. The Vanir put forward their noblest men,
Njǫrðr the Wealthy and his son Freyr, and the Æsir in return the one called
Hœnir, and they claimed that he was very suitable to be a ruler. He was a
large and most handsome man. With him the Æsir sent the one called Mímir,
a very clever man, and in return the Vanir put forward the wisest in their
company. He was called Kvasir. But when Hœnir came to Vanaheimr
he was at once made a lord. Mímir always told him what to do. But when
Hœnir was present at councils or meetings where Mímir was not nearby,
and any problem came before him, he always answered the same way: ‘Let
others decide.’ Then the Vanir suspected that the Æsir must have cheated
them in the exchange of men. Then they took Mímir and beheaded him and
sent his head to the Æsir. Óðinn took the head and smeared it with herbs
that prevented it from decaying, and recited spells over it and imbued it with
magic power so that it spoke to him and told him many secret things. Njǫrðr
and Freyr Óðinn appointed as sacrificial priests, and they were gods* among
the Æsir. Njǫrðr’s daughter was Freyja. She was a sacrificial priestess. She
was the first to teach the Æsir black magic, which was customary among the
Vanir. When Njǫrðr was among the Vanir he had been married to his sister,
for that was the law there. Their children were Freyr and Freyja. But it was
forbidden among the Æsir to cohabit with such close kin.


- Heimskringla, Ynglinga Saga, Chapter 4


("Díar (only used in plural) is a loanword from Old Irish día, related to Latin deus and Old Icelandic tívar. It normally means ‘gods’, but here may mean ‘high priests’." - Footnote included with translation)