Advice for a 9th Level Rust Spell?

[BlueWitch]

I'd like to make a Rust Spell but for the 9th Level Spell Slot,
so pack more of a punch than the 4th Level one.

The problem is I don't really know what would be too op.

Mage's Disjunction can make a magical item fall apart. But we wanted this to affect only metal (including metallic creatures like Iron Golems) and in a targeted radius.

We thought maybe 20d6 per turn (No Save) and duration to be Concentration.
How does that sound?

(Some back story is it's from an Epic Druid that destroyed an Army of Iron Golems, but we want the spell available in the campaign for flavor.)

[MaxiDuRaritry]

I'd like to make a Rust Spell but for the 9th Level Spell Slot,
so pack more of a punch than the 4th Level one.

The problem is I don't really know what would be too op.

Mage's Disjunction can make a magical item fall apart. But we wanted this to affect only metal (including metallic creatures like Iron Golems) and in a targeted radius.

We thought maybe 20d6 per turn (No Save) and duration to be Concentration.
How does that sound?

(Some back story is it's from an Epic Druid that destroyed an Army of Iron Golems, but we want the spell available in the campaign for flavor.)Actually, how about a spell that ages any non-living/non-natural things extremely quickly in a Long AoE? Animals, humanoids, plant creatures, elementals, and such are unaffected, but ordinary plants grow exceedingly fast (as per plant growth), unnatural creatures must make a Fort save or age to death (except immortal creatures, but they still age), structures take damage and start falling apart, and constructed creatures rust or rot into dust, depending on what they're constructed of.

[BlueWitch]

Actually, how about a spell that ages any non-living/non-natural things extremely quickly in a Long AoE? Animals, humanoids, plant creatures, elementals, and such are unaffected, but ordinary plants grow exceedingly fast (as per plant growth), unnatural creatures must make a Fort save or age to death (except immortal creatures, but they still age), structures take damage and start falling apart, and constructed creatures rust or rot into dust, depending on what they're constructed of.

I like this idea! Thanks for the inspiration, mate! That actually does fit with our Grand Druid's history and mannerisms alot.

[Chronos]

For a ninth-level spell, some things you'd want:
1: Very large area of effect. After all, you wanted to take out a whole army of iron golems. At least hundreds of feet radius, maybe even a mile radius.
2: Selective targeting. If you have allies who use metal, you want to be able to exclude them. Well, OK, maybe the original druid didn't care about this, but it's not worth a 9th-level slot without it.
3: Instant destruction of nonmagical metal.
4: A chance to destroy metal magic items. Probably don't want to make it too high of a chance, though: If your players ever face an enemy using this, they'll be very salty about losing all of their magic weapons and armor.
5: Damage against metallic monsters.
6: No spell resistance, or specific wording for it to get past the magic immunities of golems.

[Thunder999]

Given that Mage's Disjunction exists a custom 9th level spell that just corrodes every metal object in range to dust (fort negates for attended or magical stuff) seems reasonable to me. Not sure why you'd want to use it though, Disjunction at least has the niche of a guaranteed dispel for any annoying buffs, destroying gear just loses you loot and it's not hard to kill things that fail a save.

[Bohandas]

Maybe a reverse or transfer iption, where you could also un-rust things or transfer rustiness from one place to another

Another possibility would be optional secondary fire effects, either on the rationale of fire and rust being related reactions chemically or else on the rationale that rust is a component of thermite

[Hua]

The aging is an interesting idea, but IMO would not work for this back story.
The Constructs do not age or break down from duration. Making age/time impact them with this spell when it does not normally seems very forced.
Using time, it also opens this spell up to a lot more uses than what the backstory calls for. It turns a uniquely designed spell into a useful general spell, which happened to be useful for the backstory.

My read on your initial ask was for something specifically for the historical event. Something just for flavor. Making it as general as the suggested time spell will not be just flavor. It will be a spell in high usage by anyone that gets to 9th level spells.

A Time-speedup spell sounds great, but not for what you asked for.
I'd suggest going with Chronos' suggestion, despite that someone named Chronos should be supporting a time-speedup spell.