biome not spawning

i have a custom biome for my dimension but i dont know how to get it so it doesnt spawn in the over world here is the biomes code :

package minecrafttale;

import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;
import net.minecraft.block.Block;
import net.minecraft.entity.monster.EntityGhast;
import net.minecraft.world.biome.BiomeEndDecorator;
import net.minecraft.world.biome.BiomeGenBase;
import net.minecraft.world.biome.SpawnListEntry;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.event.terraingen.BiomeEvent;

public class valhallaBiome extends BiomeGenBase{



public valhallaBiome(int id){
super(id);
this.minHeight = 60;
this.maxHeight = 232;
this.waterColorMultiplier = 0xED0202;
this.spawnableMonsterList.add(new SpawnListEntry(EntityViking1.class, 2,1,1));
this.spawnableWaterCreatureList.clear();
this.field_82914_M.clear();
this.spawnableCreatureList.clear();
this.spawnableMonsterList.clear();
this.topBlock = (byte)mod_MainClass.valhallaTop.blockID;
this.fillerBlock = (byte)mod_MainClass.valhallaFill.blockID;
}
}