21:30:45 [SEVERE] Could not pass event DiabloMonsterDamageByEntityEvent to DiabloDrops v2.1_b318
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:341)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:477)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:462)
at com.modcrafting.diablolibrary.EntityListener.onEntityDamageByEntity(EntityListener.java:95)
at sun.reflect.GeneratedMethodAccessor977.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:339)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:477)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:462)
at org.bukkit.craftbukkit.event.CraftEventFactory.callEvent(CraftEventFactory.java:80)
at org.bukkit.craftbukkit.event.CraftEventFactory.callEntityDamageEvent(CraftEventFactory.java:377)
at org.bukkit.craftbukkit.event.CraftEventFactory.handleEntityDamageEvent(CraftEventFactory.java:399)
at net.minecraft.server.EntityLiving.damageEntity(EntityLiving.java:658)
at net.minecraft.server.EntityHuman.damageEntity(EntityHuman.java:605)
at net.minecraft.server.EntityPlayer.damageEntity(EntityPlayer.java:306)
at net.minecraft.server.EntityMonster.m(EntityMonster.java:88)
at net.minecraft.server.EntityMonster.a(EntityMonster.java:110)
at net.minecraft.server.EntityCreature.bn(EntityCreature.java:56)
at net.minecraft.server.EntityLiving.c(EntityLiving.java:1228)
at net.minecraft.server.EntityMonster.c(EntityMonster.java:20)
at net.minecraft.server.EntityEnderman.c(EntityEnderman.java:176)
at net.minecraft.server.EntityLiving.j_(EntityLiving.java:508)
at net.minecraft.server.EntityMonster.j_(EntityMonster.java:24)
at net.minecraft.server.World.tickEntity(World.java:1458)
at net.minecraft.server.World.entityJoinedWorld(World.java:1420)
at net.minecraft.server.WorldServer.entityJoinedWorld(WorldServer.java:569)
at net.minecraft.server.World.playerJoinedWorld(World.java:1410)
at net.minecraft.server.World.tickEntities(World.java:1288)
at net.minecraft.server.WorldServer.tickEntities(WorldServer.java:463)
at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:562)
at net.minecraft.server.DedicatedServer.r(DedicatedServer.java:222)
at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:478)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:413)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:856)
Caused by: java.lang.IllegalArgumentException: Health must be between 0 and 20
at org.bukkit.craftbukkit.entity.CraftLivingEntity.setHealth(CraftLivingEntity.java:59)
at com.modcrafting.diablodrops.effects.EffectsAPI.addEffect(EffectsAPI.java:170)
at com.modcrafting.diablodrops.effects.EffectsAPI.handlePluginEffects(EffectsAPI.java:411)
at com.modcrafting.diablodrops.listeners.EffectsListener.onDiabloMonsterDamageByEntityEvent(EffectsListener.java:37)
at sun.reflect.GeneratedMethodAccessor978.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:339)
... 36 more

This has been resolved in later builds. :) I did verify that it was fixed.

int chng = level - damaged.getHealth();
if ((chng < damaged.getMaxHealth()) && (chng > 0)){
     damaged.setHealth(chng);
}else{
     damaged.setHealth(0);
}