Old Norse is the name given to the language of the Vikings. There were to main types of Old Norse, Old West Norse which lead to Icelandic, Norwegian and Faroese, and Old East Norse which lead to Swedish and Danish. Most of the literature we have today is in Old Icelandic.
Modern Icelandic has changed very little compared with Old Icelandic and Modern Icelandic speakers can follow Old Icelandic with little difficulty. Norse Mythology was written in Old Norse, and learning Old Norse is a must for anyone with an interest in Norse Mythology.
Learning older languages is also a very fascinating endeavour as it allows you to see the links with modern languages. I have been learning Swedish and I have noticed some very interesting parallels with Old Norse. “kvenna” in Old Norse became “kvinna” in Swedish, which means woman, and “mikill” became “mycket” which means “a lot, much, great, many”. I very much recommend looking into the language of the vikings! Skål (said to toast good fortunes while drinking) 🙂