Stunning Strike states that:
When you hit another creature with a melee weapon attack, you can spend 1 ki point to attempt a stunning strike.
However, some creatures are immune to the types of damage (bludgeoning, piercing, slashing) that can be dealt with melee weapon attacks.
My interpretation is that an attack can still hit, but would be guaranteed to do zero damage. And assuming it did hit, an attempt can still be made to stun the creature.
Is this interpretation correct?
I agree.
The key word in the rules is "hit". Damage immunity prevents the damage, not the hit itself. So yeah, I would say that the magic nature of Ki would probably have a chance of stunning the creature.
Just wait till LVL 6, then the Monk's fists count as magical weapons for the purpose of harming creatures immune to non-magical attacks!
Yes the ki power will inflict the stunned conditioned, unless the creature is immune to the stunned condition.
The Monster Manual defines this on page 8 (Chapter 1: Statistics: Vulnerabilities, Resistances, Immunities)
Some creatures have vulnerability, resistance, or immunity to certain types of damage. Additionally some creatures are immune to certain conditions. If a monster is immune to a game effect that isn't considered damage or a condition, it has a special trait.
Immunities to damage, conditions, and other effects are separate issues.
This rules applies to all types of damage, and would not need to be an additional "part" of an attack. Even an Ancient Silver Dragon is affected by the Ray of Frost cantrip, being slowed by 10 feet. The dragon is immune to the cold damage, but not the other effect of the spell.
