Do Not Play: The Bigger They Are by Machinae Supremacy

[classicdrogn]

Oh yeah, this would definitely be a down-beat if it came up - first off losing an ally in the first place is usually a darker moment for most stories, then there's the hope spot of "but wait, maybe we can still turn this back..." but instead it's a boss fight against the face of a former friend, who knows just enough of the shared history to really make it hurt without being susceptible to being talked around. Definitely something to go in the stage when things look bleakest, before the heroes find some way to save what remains and build anew in the broken places, even if what was lost can not be regained in its own right.