You may wish to take some time to level up here. Do so in the first screen of the desert, or if you find it too difficult, get on the boat and head back West. You won't need to fight enemies on the way back, just come back East when you're done.
In the desert, you'll find cactus fruit on some cacti. It's not much good - it'll give you 50 HP most of the time, but sometimes it drains 50 HP if it's a bad one. But collecting them can earn you the Gimme the Loot! achievement (find 50 treasures) and there's another achievement for getting sick off of eating them.
Follow the directions on the Rune you got from the sea Monster. After going West, find the chest along the edge of the map with the Ice Sickle in it.
At the end of the final section of desert, a Druid will give you your second Vampire - vIce. Just North of here is Crossroads Castle.
