A reflection is only a variation of you that is paler.
Chrom dies by Grima's magic.
Chrom is brought back from death by Grima's magic.
He is not brought back living, but risen. His skin is gray with the pallor of death, and his eyes are red now. He looks to Grima like he looked to Robin in life, with trust in his eyes.
"We shouldn't be going to war alone, Chrom." Grima suggests softly through the human avatar. "We should bring our friends into this."
The Shepherd's fall to Chrom's blade, and are brought back by Grima's magic. Not all of them, not right away, some escape, some die to other causes, some have their corpses successfully burned. But enough of them come back as risen to still count as the Shepherd army.
The Shepherd Army is led by Chrom, and directed by Robin. That has not changed in death, save that Robin is Grima now.
Grima places a crown upon Chrom's head. This one is different from what he wore in life, spiked and dark, belonging to Grima.
In life they fought war after war, a perpetually never ending cycle. Warmonger Chrom, they called him, a stark contrast to his sister, Peacekeeper Emmeryn. Chrom was a man who had wanted peace, but had no means to achieve it, the legacy left by his father swallowed the
world his sister had tried to make better. Robin had left his father, and ended up on the other side of a war against his country. Robin followed Chrom into it, and into the wars that followed.
Life had been a series of wars, and death was all one big war against everything else. All the war has only honed their abilities to fight better, as they follow Chrom and Grimia into the fight against the living.
In many ways, nothing has changed. Life is meaningless, death is meaningless, and yet the ties that bind them still hold true.
Death can no longer break them apart, but unites them, stronger than ever.