How does grim aura work?
 

Is it only beneficial to empty hands or is it something that works with 2x1hb?
is it worth killing grachnist the destroyer (spelling) for pre-monk epic?

Grim Aura applies a flat +Attack Power buff. Attack Power increases your damage on every hit.

Atk makes you more likely to hit for max damage. It's a small boost but worth having on all the time as a melee. One of the best reasons to have an earring though is as a junk buff. Keep that top slotted all the time so dispells hit that instead of your real buffs... well before they completely broke how dispells work here.