Is Spiritual Weapon considered a Spell or a Weapon for Resistance/Immunity purposes?

Question

In 5th Edition D&D there are various monsters with immunity or resistance to magic. For example, the Rakshasa is immune to spells of 6th level or lower. Many of these monsters are not also immune/resistant to magical weapons however.

If the monster is attacked with Spiritual Weapon would the immunity/resistance to magic still be in effect?

On the one hand it is a spell which attacks with a "melee spell attack" (phb 278), on the other hand the spell explicitly says it creates a weapon (although "spectral").

Is it merely a spell in the shape of a weapon (and therefore immunities will work on it) or does it conjure a temporary magical weapon (and therefore magic immunities will not work on it)?

Answer 1

Making the answer as simple as possible: a snippet of Spiritual Weapon description:

Clerics of deities who are associated with a particular weapon (as St. Cuthbert is known for his mace and Thor for his hammer) make this spell's effect resemble that weapon.

It is not a weapon. It is a 2nd level spell whose effect resembles a weapon. The Rakshasa is immune.

Answer 2

Each of these has to be decided on a case by case basis and ultimately this is a DM call.

For Spiritual Weapon the evidence is that the Rakshasa is immune¹:

1. The damage comes from a "spell attack": this is pretty much the clincher, but also,
2. It is an Evocation spell: evocations "manipulate magical energy to produce a desired effect"; contrast this with Conjuration which "involve the transportation of objects" or Transmutation which "change the properties of a ... object",
3. It does force damage; weapons typically do bludgeoning, piercing or slashing damage.

¹Assuming that the caster did not use a 7th level or higher spell slot to cast it.

Answer 3

It is a spell in the form of a weapon. It is susceptible to magic immunity.

Magical weapons are usually items. In instances where weapons can be imbued with magical properties in relation to damage resistance that is clearly stated. (Shillelagh PHB 275 - "The weapon also becomes magical, if it isn't already" or Pact of the blade - PHB 107 "This weapon counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage").

Moreover the Spiritual weapon deals force damage plus the caster's relevant ability modifier, implying that its' potency is relevant to the caster's magical aptitude, not his Strength or Dexterity, just like a spell.

Answer 4

Bludgeoning, Piercing, and Slashing damage from spells bypass resistance/immunity from non-magical weapons

Even if the weapon seems mundane, but because it is conjured by a spell, it is magical and therefore pierces resistance/immunity.

However, for the Rakshasa and other creatures with Limited Magic Immunity, it is immune to all spells of 6th level or lower, so if you cast Spiritual Weapon at 5th or lower, it would be immune to it.