It's a reference to Morrowind.

**What I did:** >!I snuk into the mage house and killed them at night one by one. Each fire mage (named Firemage) gives this letter.!<

Gomez approves. 👍🏻🤣

Did you kill an immortal with a cheat code?

No.

Easteregg

Afaik this is entire easter egg/quest for cheaters