There's no Steam news item announcing this 75% off sale for Trine and no indication as to when this sale ends. So I'm not sure when the special started or when it will be over.
Trine is a fantasy action game where the player can create and use physics-based objects to beat hazardous puzzles and threatening enemies. Set in a world of great castles and strange machinery, three heroes are bound to a mysterious device called the Trine in a quest to save the kingdom from evil…Lashiec says Trine would go in his Best Games of 2009 List
The gameplay is based on fully interactive physics - each character's different abilities help the player battle an army of undead and defeat hazardous contraptions. The player can at any time freely choose whoever is best suited for the upcoming challenge or puzzle: The Wizard is able to summon objects to help solve puzzles and create new ways to overcome obstacles, the Thief uses her agility and dead-on accuracy to swiftly surprise the monsters, and the Knight unleashes mayhem and physical destruction wherever he goes.
even after only having played the demo.
Trine on Steam also has Steam Cloud support, meaning your save data is saved in the Cloud and available at any PC you log into your Steam account from.