Board Thread:General Discussion/@comment-11276487-20130714031200/@comment-13199219-20130725051333

Numidium is mentioned in many but makes an appearance in Daggerfall, whilst the Heart of Lorkhan is also mentioned in many, but is seen in Morrowind. (Elder Scrolls 2 and 3 respectively)

There are several in-game books in Skyrim which might clarify some other questions you may have.