I'm supposed to do a quest from abdullah called "Assassin's Duel" so i can do himm chamber.. but i can't get the quest and error pops instead "This item cant be reverted"..
is the quest buf atm? or have i missed some quest before this one.. please help
It sounds like it's not giving the Letter you're supposed to deliver. Try sending a ticket to support.warpportal.com, maybe with some extra info, devs will be able to fix an issue if it's still there.
Optionally, you can try asking them to activate that quest in your character and send this: http://ro2.doskoi.org/item/40001163 to your bag so you can complete the quest.