The active effect on Entropy is the only active effect on any item that procs the bonus damage from Sheen, Trinity Force, and Lich Bane.
Entropy is designed for a front-line fighter to stick to his target on the smaller maps, where short-range dashes and blinks are very strong. The active effect deals damage over time, which can secure a kill on an enemy with an escape.
Frozen Mallet is an alternate Legendary item which performs a similar function. It has less damage but a strong slow and more health, making it better on melee fighter champions who need more health to survive being in close-range and a consistent slow to lock down an enemy.