20it is a prank, not a weapon
You throw it and it breaks. The damage it does belongs to no class at all, so nothing in your build makes it hit harder and there is no version of a fight where you would want it in your hand. What it really does is leave a patch of bad luck on the ground where it shatters.
the field does not care who threw it
Anyone standing in the blast picks up the penalty, thrower included, and the area itself does no damage, only a direct hit does. In multiplayer that makes it a griefing tool and close to nothing else. Alone it is a way to accidentally ruin your own fishing session.
getting rid of it
A second hit refreshes the timer instead of stacking, so being caught twice makes it longer, not worse. It burns off with playtime, and anything that speeds the clock speeds it up too: sleeping, the sundial, the moondial. Leaving the world clears it outright. Dying does not.
seven minutes
The duration is a shrunk down version of the Roman superstition about seven years of bad luck for a broken mirror.