"Invitation to Morrowind" is a Letter (mail) you get that starts the quest line for Morrowind for existing characters.
The letter reads:
Starting the Morrowind Quest
There should be an attachment to the letter and taking that attachment will start the quest, Divine Conundrum. However there are reports this is bugged and only works for first character you log on with. The alternate way to start the quest is to open the COLLECTIONS menu and choose STORY and go to MORROWIND and click on "ACCEPT QUEST". See screenshot below:
- You can only start this quest if you bought the Morrowind DLC.
- Instead of opening the letter attachment, or starting the quest from your collection tab, you can instead just travel to Vvardenfell yourself and talk to either Alavesa Arethan or Canon Valasa around Seyda Neen
- New characters you create will start at Vvardenfell by default if you own the Morrowind expansion.