The game has been made available for free by Croteam, the developers of the Serious Sam series, on April 14th, 2022. This version is a complete standalone expansion, and you can download it from various sources.
Please note that while the game is free, some features or content might be limited compared to the full Serious Sam games. Serious Sam: Tormental Free Download (v14.04.2022)
You're looking for a free download of Serious Sam: Tormental, a standalone expansion to Serious Sam 2. Here's what I found: The game has been made available for free