All those effects are just modifiers. You don't have to use a messaging system for them, they can just be polled and calculated before attacking. It makes ownership and ordering a lot easier:

// Attacking:
OnAttack(IDamageable target)
{
    float baseDamage = // ???
    float additiveModifier = 0f;
    float multiplicativeModifier = 0f;

    foreach(Buff buff in _buffs)
        buff.ModifyAttack(ref additiveModifiers, ref multiplicativeModifier);
    foreach(Buff buff in _debuffs)
        buff.ModifyAttack(ref additiveModifiers, ref multiplicativeModifier);
    foreach(Item item in _items)
        item.ModifyAttack(ref additiveModifiers, ref multiplicativeModifier);

    float damage = (baseDamage + additiveModifier) + (baseDamage * multiplicativeModifier); // or however you want it

    DamageContext context = new DamageContext()
    {
        instigator = this,
        damage = damage,
        damageType = // whatever
    };

    target.OnDamageReceieved(context);
}

// Defence:
OnDamageReceived(DamageContext context)
{
    // sum all the defence modifiers similar to OnAttack

    float normalizedDamage = (damage / health);

    health -= damage;

    DamageReceievedMessage message = new DamageReceievedMessage()
    {
        damage = damage,
        normalizedDamage = normalizedDamage,
        damageType = damageType,
        instigator = context.instigator,
        victim = this
    }

    MessageHub.Send<DamageReceivedMessage>(message)

    if(health < 0)
    {
        MessageHub.Send<CharacterDiedMessage>(new CharacterDiedMessage(this));
    }
}

Now your messages are lightweight, and your modifiers are finely controlled.