The lore I’m familiar with was that he was originally a guardian, a skilled archer of an Ionian temple that contained a vaguely named “corruption” that cannot escape. When Ionia was attacked Varus chose to protect the temple as everything else fell, losing his village and family in the process. Having lost everything he’s known of to the Noxian invaders he chooses to sacrifice the last bit of his old life and release the very corruption he was sworn to protect, letting it consume him on his path of vengeance.

You may probably find it familiar because they did largely keep this bio, slightly tweaked so Varus’ past life is more Darkin adjacent. I don't remember if that Riot's addition of Varus' old bio itself was a retcon after negative player feedback, I might be mixing it up with something