Can I play the expansion Eternal Embers on normal difficulty?
The new Act is only available in Legendary difficulty. I believe there's a new character creation option (~Legendary character) that starts the char off at the start of DLC in Legendary with some gear to get going. Otherwise you'll have to level a character through normal/epic/act1-4 legendary.
The new mastery introduced in DLC is of course available for any characters with a free mastery slot so if you want to try the new mastery you can start a new character and play mastery.
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Kill Hades or Surtr on Legendary to get a portal to take you to Embers.