Bukkit

import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerMoveEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class DeathByVoid extends JavaPlugin implements Listener {
	public void onEnable() {
		getServer().getPluginManager().registerEvents(this, this);
	}
	
	@EventHandler(ignoreCancelled = true, priority = EventPriority.MONITOR)
	public void onVoidEnter(PlayerMoveEvent e) {
		if (e.getTo().getBlockY() < -1)
			e.getPlayer().setHealth(0.0);
	}
}