Many sites claiming to offer free downloads bundle the files with trojans, ransomware, or miners that can steal your data or ruin your PC. No Official Support:

Many sites claiming to offer "cracked" versions of the game actually bundle installers with trojans, miners, or ransomware that can compromise your PC and personal data.

It is important to be cautious when looking for "gratis" or free full-version downloads of on PC. While the game has been out since late 2023, seeking it through unofficial channels carries significant risks. The Risks of "Free" Downloads

During major sale events, Epic often provides additional discount coupons that can be applied to Mirage. PC System Requirements

That being said, here's some general information about Assassin's Creed Mirage: