I'm new in the game. Can someone tell me what does this item corrupt exactly? It says "corrupt the user", does it change anything, it's better not use it, or....?

Murphy's Hand is a Newbie-trap: Don't use it. Your Hero gains 1 point of corruption each turn in combat. 

It's basicly a last resort item against very difficult bosses. But i can asure you that it's possible to beat all bosses in the game, even optional bosses, without Murphy's Hand. High Endurance [Opinion; 7-8 END for high-STR builds and 10 - 12 END for low-STR Builds] and resulting high Defense (20+) is the key to win even the toughest Boss battles.