How to properly start your first chapter with a fast-forward scene?

I already wrote a lot of my draft, three books and half of the fourth. The first one starts with a fast-forward chapter of the present times (the year 2023 of our planet Earth) and then goes back to the 9th century of Iceland, where the main cast starts as Vikings; they become immortals and live through ages till they reach the 21st century, but this does not happen till chapter 20.

I did this to make readers know how the story goes without a lot of spoilers, so they do not mistake my book for a simple historical Viking tale but a fantasy one that is set on earth.

Do you have advice for me on how to do this the proper way?