What's the first fief (Feudum) in the history? And what was their features?

You need to make your terms clearer. Go back and reread your notes, failing that: ask your teacher for clarification.

The mystery for us is what exactly you are speaking about.

Start here: https://en.wikipedia.org/wiki/Feudalism

This talks about a second feudal age: https://link.springer.com/chapter/10.1007/978-1-349-00253-5_2

Home: The History of Feudalism

The Second Feudal Age

David Herlihy

Bobsal's right, you're incredibly vague.

What do you mean by second fief?

A you talking about a country, a region or an entire continent?

When are you talking about, is this medieval, classical or prehistoric, because all three had such systems?

Bad enough they want us to do their research for them, but when they don't bother to get the assignment clear in their brain pans ...

It's especially important to know - time and location.
Fiefs, holders of allods, feudatories, vassals ... all that is different not only by the word.