He wants you to steal three items needed for the creation of Halruaan Skyships. These items can be found on the third floor of the Estate of Oberon, in Baldur's Gate Central and are guarded by three Wizards.
If you enter the premise unarmed (fists selected as weapons with all characters) and talk to Glanmarie you can claim to be a servant (dialog options 'new position' and 'no experience' will do the trick, though she will summon guards if she becomes suspicious). Oberon on the second floor will ignore you, as long as you are unarmed. On the third floor you will be hailed by one of the three wizards. They will get suspicious of you and subsequently fighting will break out, which is bad for the wizards. The three items needed for the quest are stored away in the drawer and the cupboard. Nobody else in the house will have noticed your violent slaughter of the wizards so you can leave undisturbed.
Talk to Alatos again. Resar will take the items off you (4000 XP) and then try to kill you as you are expendable to Alatos and a loose end for Resar. Only Resar will become hostile, as long as you don't hurt any of the thieves, and he can be killed without any loss of reputation concerning the Thieves' Guild.
DISCLAIMER: If you completed both quests given to you by Narlen beforehand, he will backstab Resar (in a cutscene) and no fighting will take place. Alatos, confronted with the prospect of mutiny of some of his thieves, will agree to spare you.