After you get the Shield of Athena in Athens, you should return to Eleusis and defeat the Lamia.
Go to the Lamia
Leave Athena's temple and go to the left until you reach a dead end. Go through the door to return to Eleusis. From there, go to the left until you find some stairs going down. Go down those stairs, then go to the left until you find a hole in the ground. Jump over it and slowly go to the left. Lamia will slowly slither toward you.
If you just stand there, Lamia will shoot arrows at your shield, and they will bounce off (assuming you got the Shield of Athena earlier). In order to hit Lamia, you should wait for an arrow to hit your shield, then immediately run toward her, jump to avoid her next arrow, hit her, then run away, jump to avoid her next arrow, then turn around to block her next arrow with your shield. It may take some practice. If you die, be sure to choose Retry so that you will restart at the last place where you got a password from a God and you won't lose any olives.
After Lamia dies, go into the cave that appears, and talk to the boy.
Get the Staff of Fennel
Leave the boy's cave, then go to the right to return to the area with white houses. Go into the upper-left house to find the boy's mother. Talk to her and she will give you the Staff of Fennel.
The Staff of Fennel is more powerful than the club, and can defeat enemies in fewer hits. Be sure to press START and choose the Staff of Fennel and press B or A to equip it.
Now that you have the Staff of Fennel, you need to visit Prometheus, who will add the power of fire to it.