Today Paradox Interactive announced the release date of Imperator: Rome, scheduled for the 25th of April 2019.
If you own Imperator: Rome (pre-ordering counts as owning) and Europa Universalis IV you'll get this Roman Unit Pack for Europa Universalis IV.
https://store.steampowered.com/app/859580/Imperator_Rome/
To activate this offer you need to have a Paradox Account linked with your Steam account. You can find information on how to do that here!
*This offer will be valid also after the release of Imperator. This is not pre-order exclusive