D&D 5E Necrotic damage blade against physical structure?

[Voadam]

So in my 5e conversion of the sci-fi fantasy mashup Iron Gods adventure path an AI monster truck demigod that took on a demon persona empowered one of its orc champions with an unholy chainsaw with a phantom demonic chain instead of a physical metal one so that it would not snap on contact with metal and such. She used it to cut opponents in half and lop off heads.

Mechanically essentially a two handed sword +1 that does necrotic instead of slashing damage.

So now the PC hexblade has it and made it his big weapon after losing his hand to a crit from the orc before killing her. Big Army of Darkness vibes.

I have this all loaded into FG so it mechanically is already not damaging things immune to necrotic damage as has come up in the game and the PC is taking into account.

The PC now wants to try it out against the metal bars on a window in a wizard's abandoned tower.

Thematically what should be the result? No effect because necrotic energy or should it demonically chew through the metal?

 

[the Jester]

Why wouldn't necrotic energy damage metal? It's the power of decay. The metal warps, rusts, weakens, and crumbles.

 

[Oofta]

Up to you of course. Typically necrotic doesn't damage physical material because it's disrupting life force or animating spirit. If you want it to work, it just chews things away like an acid, leaving pitted stone, corroded metal.

 

[mellored]

RAW
"Objects are immune to poison and psychic damage.".
So Necrotic would work.

But also
"As always, use your best judgment."

Personally, I would split the difference. Give it Resistance.

 

[Reynard]

Why wouldn't necrotic energy damage metal? It's the power of decay. The metal warps, rusts, weakens, and crumbles.

This is the way.

 

[Tonguez]

Necrotic = decay, metal decays just like flesh does. Personally I’d treat it like an acid effect