Ygtjki

The description for spiritual weapon states that (emphasis mine)

You create a floating, spectral weapon within range that lasts for the
duration or until you cast this spell again. When you cast the spell,
you can make a melee spell attack against a creature within 5 feet of
the weapon. On a hit, the target takes force damage equal to 1d8 +
your spellcasting ability modifier.

As a bonus action on your turn, you can move the weapon up to 20 feet
and repeat the attack against a creature within 5 feet of it.

If there is an illusion of a creature in the path of where you want to move the weapon what happens?

1. The weapon cannot pass through the illusion so would have to move
   around it


2. The weapon in spectral form passes through the creature whether or not it is an illusion but does not reveal it


3. The weapon passes through the creature because it is an illusion which then reveals it as an illusion


4. Something else?

I think:

4. Something else?

There is not much RAW specific to Spiritual Weapon movement.

However, we do know that:

• Spiritual Weapon is not a creature, it is a spell effect



• 
  

  It does not occupy the square that it is in, allies may be in the same place for instance. This is based on:

  
  
  
  • Occupying space is not included in the spell description
  
  
  • Spell effects don't usually occupy a space unless stated
  
  
  • Weapons don't usually occupy a space
  
  
  • Most weapons would convert to size Tiny in any case, and be able to share space with a Medium creature
  
  



• It cannot be the target of an attack itself, so does not trigger opportunity attacks etc

As such, I would rule that it can move relatively freely through creatures' squares without interacting with them. With only 20' of movement to allocate, it doesn't seem fair to make it take the long way around if it doesn't need to.

Spiritual Weapon does not appear to need to interact with any creature, object or illusionary creature in a square that it could pass though. The only interactions allowed for in the spell description are its attack and visual appearance. In fact it is not even specified that Spiritual Weapon actually traverses the distance, or just appears in a new location when you move it. I have always imagined it traversing, but given the lack of interactions I don't think it makes any difference which description you give it.

Further to that, I would treat any attempt to deliberately "go through" a creature as the designated attack for the spell that turn - it would end its movement and make an attack roll. If that would reveal a target as an illusion under normal rules for the illusion effect in play and any other melee attack, then I would expect the Spiritual Weapon interaction counts in the same way.