The term "Mega" usually refers to , a popular cloud storage service. Users often compile "series" or "packs" of content and share them via encrypted folders. While the "free" aspect is the main draw for many, these links often act as gateways to more than just media files.
To access "free" content, some sites will ask you to "verify your age" or "create a free account" by entering credit card details or email addresses. These are almost exclusively phishing attempts to steal financial information.
Many sites that claim to host "mega packs" or "free series" are laden with aggressive pop-unders and scripts. Clicking a "Download" button may trigger an executable file (.exe or .scr) disguised as a video file, which can install keyloggers or ransomware on your device.