Trial of Gravity Submission For the turn that Shard of Mercy is on the field, creatures that would die are instead moved to their owner's hand, and deal damage to their owner equal to their attack. In other words, your creatures come back to your hand, fully refreshed, but attack you in the process. This might seem like more trouble than it's worth, but I designed this card with synergy with Rain of Fire and Thunderstorm in mind. Let's say you're facing a VNG with a Crimson Dragon, two fresh Lava Destroyers, and a Phoenix. You play your Shard of Mercy, then play a Rain of Fire. Since it deals 3 damage to all, it
would kill all four enemy creatures. But because the Shard of Mercy is on the field, all four creatures are rewound back into the enemy's hand instead. He loses a total of 12 + 7 + 7 + 7 = 32 life. Next turn, he can play them all again (if he has the quanta to play them), but that's if he survived the 32 damage.
The Shard of Mercy affects you as well. Let's say your opponent also has a Rain of Fire, and he uses it on your creatures. Maybe you're playing a CCYB variant, and all your Fireflies are "killed". Let's say you had six of them, and they're all moved to your hand. You're dealt 24 damage. Now this can be a double-edged sword, but the effect can be lessened by choosing when to use the Shard and with what sort of deck.
Now these cards are moved to your hand, not rewound. What if you don't have enough space for your hand? Then the creatures will simply die. In the first scenario, let's say your opponent had five cards in his hand. The Dragon and both Lava Destroyers are moved to his hand, but by now his hand has filled up. The Phoenix can't make it in, so it simply dies -- or in its case, turns to ash.
As for the benefit to Gravity, most Gravity creatures have low attack. Playing this in a mono-Gravity would result in minimal risk to yourself, as they would damage you for far less.