[SPOILER] Engineer's last speech

[Froge]

I have stood knee deep in mud and bone, and filled my lungs with mustard gas.

WW1 soldiers fought in trenches and were frequently exposed to mustard gas, a deadly chemical that formed blisters on skin and lungs.

I have seen two brothers fall. - Which brothers?

Definitely just referring to Mandus' sons.

I have lain with holy wars and copulated with the autumnal fallout. - The crusades?

Just holy wars in general from past, present, and future. Those are still going on in our world.

I have dug trenches for the refugees

Digging trenches for the refugees means the refugees are probably going to die. During Operation Barbarossa in WW2, German soldiers frequently lined up Russian civilians in front of trenches and then shot them in the back of the head so that they would automatically fall in.

I have murdered dissidents where the ground never thaws, and starved the masses into faith. - Religious oppression?

Pretty much.

A child's shadow burnt into the brickwork. - What child?

Any child that died a horrible, fiery death would do. And those were pretty common in gruesome 20th century.

A house of skulls in the jungle. - Aztec massacre?

Pol Pot or Vietnam war. One of the many massacres that took place in Southeast Asia.