I have been assured that this is not in fact an April Fool's joke, and thus I deliver the news that you may now opt to replace the smooth and soothing voice of Elohim, the godlike overseer of The Talos Principle, with the rather rougher tones of Serious Sam.
It will still be Elohim's words and wisdom guiding you through the existential maze that makes up your journey through The Talos Principle, but the new DLC pack means they will delivered by Serious Sam voice actor John J. Dick. The DLC also includes a new Serious Sam player model, for those who'd prefer the game to be a little less serious. (Which is ironic, I know.)
Injecting one of the ultimate embodiments of the thick-headed FPS macho man hero into a game that's essentially a slow-paced meditation on the meaning of life, consciousness, and personhood might seem counterintuitive, but that dichotomy has been one of my favorite things about The Talos Principle from the very beginning. That the studio that gave the world "No Cover. All Man" would conceive of a game like this in the first place is flat-out weird. That it would do such a good job of it almost beggars belief.
The Serious Sam Voice Pack DLC is free until April 7, after which it will be available for purchase for $3. The Talos Principle itself, along with all other DLC, is on sale on Steam for half-price (that's $20) over the same duration.