my problem is that addInformation is not used.

import net.minecraft.entity.Entity;
import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.*;
import net.minecraft.util.Hand;
import net.minecraft.util.text.ITextComponent;
import net.minecraft.util.text.TranslationTextComponent;
import net.minecraftforge.event.entity.living.LivingDeathEvent;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import java.util.List;
import org.jetbrains.annotations.NotNull;
import static de.chigoma333.ringofblood.core.init.ItemInit.RING_OF_BLOOD;

public class EventKillCounterHandler  {
    @SubscribeEvent

    public ItemStack onLivingDeathEvent(LivingDeathEvent event) {

        ItemStack heldItem2 = null;
        if (!event.getEntity().getEntityWorld().isRemote) {
            Entity killer = event.getSource().getTrueSource();
            if (killer instanceof PlayerEntity) {
                Item heldItem = ((PlayerEntity) killer).getHeldItem(Hand.MAIN_HAND).getItem(); //MUSS ZUM SLOT GEÄNDERT WERDEN
                if (heldItem.equals(RING_OF_BLOOD.get())) {

                    heldItem2 = ((PlayerEntity) killer).getHeldItemMainhand();
                    CompoundNBT tag = new CompoundNBT();
                    if (heldItem2.hasTag()) {

                        heldItem2.getTag().putInt("KillCounterRing", heldItem2.getTag().getInt("KillCounterRing") + 1);
                    } else {
                        tag.putInt("KillCounterRing", 1);
                        heldItem2.setTag(tag);

                    }
                }
            }
        }
        return heldItem2;
    }

    public void addInformation(@NotNull ItemStack heldItem2, List<String> lores, List<ITextComponent> tooltip)
    {
        if (heldItem2.hasTag())
        {
            tooltip.add(new TranslationTextComponent("toolinfo.ringofblood.ring_of_blood"));
            lores.add("Kills: " + heldItem2.getTag().getInt("KillCounterRing"));
        }

    }
}

 

Yes, because it is a method that belongs in an Item class. Here it exists as some random method that you never reference.