You get this quest from Nadarin. He's found in the warehouse just south of Gantolandan's House in Baldur's Gate Southeast. He'll ask you if you can kill the greater basilisk that dwells in the northern warehouse at the docks.
Killing the Basilisk Edit
To kill the basilisk, you can use a Potion of Mirrored Eyes, the Protection from Petrification spell or items that provide a bonus to save vs petrification rolls, like Adoy's Belt, the Claw of Kazgaroth or protection rings or cloaks. It may be best to put protective spells and items on a fighter and have other party members attack with ranged weapons to minimize the risk of petrification. Once the basilisk, is dead you can collect a sphene gem from it that can be used to complete the quest G'axir the Seer.
After you have killed the basilisk, speak to Nadarin again to claim your reward. If you killed the basilisk before getting the quest, you can still collect the reward by talking to him.