The first step is to blind the Lich King. He's using a device known as The Ocular to watch the area, which floats above the vault's spires.
Our goblins have created something for you to use against it. After the 'eye' has been destroyed, find Baron Sliver on the vault grounds. He's pretending to still be under the Lich King's will.
The Ocular still stands! Koltira was a fool to trust you.
Whoever you are, don't return again until it is destroyed. My position here must not be compromised.
Perhaps now we can really get something done. Don't be fooled though, the Lich King can still 'see' through his Scourge minions.
That's why you're here.
The Ocular is situated above the spires of the Shadow Vault at [44.3, 21.5]
The blaster has a range of 80 yards, which is the ocular's aggro radius. It will take 10 shots of the blaster to kill it.
You can shoot the blaster from outside the Ocular's attack range, but people on Wowhead have mentioned not getting credit for the quest if they were at longest range when it died. Take it down to 12,500 hp from range, then move in and take one or two hits to fire the last shot.
Baron Sliver is just to the south of the vault at [44.1, 24.7] on the ground.
The name of the quest comes from the expression "It's all fun and games until someone loses an eye.". This expression was also a quote of C'thun.