Eluvians can be made on Thedas though... nothing is stopping physical objects from being taken into the Fade.
That's why it would have taken so much magical power to do it.
Whoever got that Eluvian into the Fade... did it the same way the Tevinter breached the Fade to get into the Golden City either 1) with a motherlode of lyrium or 2) blood magic and plenty of it.
Jowan - the worlds worst mage - even breaks into the Fade and allows you to physically enter. So I'm not sure if it's really THAT hard.
NOTE: This argument is largely academic... The Fade represents thought and is a place purely of the mind - the Golden/Black City is metaphorical and represents "Sin" which is why everyone sees a Black City because ain't nobody clean enough to see a Golden One.
In a very real sense - the "the City" is "the Self" and what you find inside of you. Nobody can ever reach the Black City because you cannot actually know yourself so well.
So... when the Magisters supposedly do it - they find only their own corruption. Becoming Darkspawn is just a phsyical manifestation of what they always were.
That's how I see all this metaphysics anyway.
The "problem" comes when Thedosians... from Solas to the Chantry - act like any of it is "real" stuff.
NOTE 2: I think the Seekers actually go into the "Golden City" during their year of extreme asceticism and then tranquil/spirit experience.
Again... it's not a real place... it's an inner place and for a moment they are purified.
Which is why they have control over lyrium and magic.