When the player rolls a d20, the Mark of Mercury allows the player to turn the die to any result without spending an action. This can be done at will, as often as the player wants, with no limitations. The caveat is that each time the player uses this ability, the DM (me, bwahaha) can do the same thing to any d20 roll - the player's or the DM's.
For evil DMs, this is a really fun trap, and potentially a very tough decision for players. If the player wants to, he or she can crit on every single attack for an entire combat - but that's a big stack of dice the DM can use against them later, or even during that fight. I think the general reaction of my players, upon receiving the Mark, was "that's really cool and I'm never ever going to use this unless I really absolutely have to".
Where this item gets really interesting is with death saving throws. If your result on a death save is 20 or higher you immediately spend a healing surge and regain consciousness - you go from dying to up and fighting on a single lucky roll. My players can now do this at will... if they're willing to risk an unexpected 1 or enemy crit at exactly the wrong time.
A patient DM doesn't have to use their end of the item in the same fight, or even the same day, as the player's use. You could have an in-game year pass before you actually use it. The trick, however, is to not be too evil or spiteful. Ideally it's best used to create a complication or plot twist, and not to just kill a character because haha evil. You might use it to make the players fail a critical time-sensitive mission, screw up a fragile diplomatic arrangement, or ensure the success of an important step in the bad guy's plan. It's a fun item for evil DMs, but there's so much potential for real intrigue and development.
It'll be fun to see if and when my players use the Mark of Mercury, and what the consequences will be when they do!