Therium-2 is now an official part of Overgrowth for the Steam Winter Sale.
You no longer need to download Therium-2 from either here or its Steam Workshop version; Overgrowth automatically includes Therium-2 by default.
Therium-2 has been remastered and includes new levels.
Every single level looks better, plays better and includes new bits of lore. Wolfire helped me improve and create new content, including a remake of the Empry, and a new tutorial level.