The Channel Divinity: Path to the Grave ability states that it makes the target Vulnerable to the next attack from an ally that hits it. If the creature were immune to such damage (EG, an Iron Golem being hit with a regular sword), how do these abilities interact?

• is the golem vulnerable to the damage and no longer immune?
• Is the golem Reistant because the Vulnerability drops it by one?
• Is the golem still immune, as it takes double damage then ignores it all?
• Is the golem neither, as they cancel out?

Thanks!

If a creature has immunity to a damage type, it won't take any damage even if it has vulnerability as well. It basically takes nothing of the damage that would have been doubled.

Here's a guideline on priority application of these rules in XGE;

Resistance and Vulnerability: Here's t he order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability. Even if multiple sources give you resistance to a type of damage you're taking, you can apply resistance to it only once. The same is true of vulnerability.

A target that's invulnerable to something is always invulnerable, regardless of effects that lower resistance - unless the spell or ability states otherwise.

There's a handful of spells and a Bloodhunter curse off the top of my head that mention it specifically turns invulnerability to resistance.

Understanding that mathematically is very simple.Vulnerability is damage*2. Immunity is damage*0. Then if you have vulnerability and immunity, it would be damage*2*0=0.

Over the years as a DM I have come across many players who have found it easier to understand this way.

Resistance vs vulnerability is trickier. I also apply it mathematically: damage*2/2=damage. However, RAW is not exactly like that since in D&D there are no real numbers (only integers). In the PHB it tells you that every time you have to divide a number, round down. Then with odd numbers you would always lose damage by having to apply resistance first (for example, if the damage is 3, after applying resistance and vulnerability, the damage would be 2. Trunc(3/2)*2=2).

I don't do it that way because that rule doesn't make sense to me. But that's homebrew.

Now say you have resistance to Poison then get Hallow to give you Vulnerability to Poison, would it cancel out the resistance to normal damage?

Quote from Trevon07 >>

Now say you have resistance to Poison then get Hallow to give you Vulnerability to Poison, would it cancel out the resistance to normal damage?

 Yes. Besically the rules  suggest this order of operations as one of the 10 core rules to remember in Xanathar's Guide to Everything:

Resistance and Vulnerability: Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability.