what does internal trauma damage do?

[Nuggs]

I see it on a number of weapons but im unsure what it does.

[Geir]

https://grimdawn.fandom.com/wiki/Damage_Types - Got most information here, is like a added damage type.

[Liquid ID]

Essentially it is the damage over time version of physical damage.

[SchnitzelTruck]

It does damage

[Raziel]

Like any other DoT, it ignores armor - but it is not stackable with itself (if from same source, but if you have Internal Trauma coming from different skills, they do stack).
Basically it is good to have against heavy armored enemies.

[Mochan]

It's just DOT Physical damage.

Damage is either Instant or Damage Over Time.

Fire is the normal "instant" damage type, its DOT version is Burn.
Lightning and Electrocute.
Acid and Poison, etc.

Bleed is its own thing though it is not Physical DOT Damage, keep that in mind. Aether and Chaos also don't have DOT variants.