Complex sentence: His name means "he who arises in might" and he was the first of the Ainur to be created by Eru Ilúvatar and he was also the one who created the discord in the Music of the Ainur.
Split into simpler sentences: His name means "he who arises in might". He was the first of the Ainur to be created by Eru Ilúvatar and the one who created discord in the Music of the Ainur.