Here's some brainstorming... I don't want to do too much backstory-- I like to discover in play. But I think this might be enough to get going... I am 100% open to revision and editing-- etc...
---
Worked the docks of a busy city. A big burly guy, good at loading and unloading shipments.
Sometimes the local navy would have him help "recruit" the local riff-raff into the service to the king's/queen's fleet. Essentially a kidnapper and pressgang work.
He got pretty good at it. Folks around the docks would treat him with a measure of respect, given the rumors--- but in truth, Ognimir only snatched up people he thought were dangerous or trouble, and never made it personal.
One day, agents of the Arcane Order arrived in the city. They were tracking a known criminal, who was about to steal away on a ship. They paid Ognimir to keep an eye out, and if he spied anything fitting the bill to grab up the criminal.
Of course, it happened, Oginimir spotted someone fitting the description, he ambushed them, went to sap and grab him-- but then things went sideways. The target cast a spell. Ognimir's skin boiled, he was afflicted. The target escaped.
The Order examined his hideous wounds. There was nothing to be done. The skin of his arms and hands are unnatural, disturbing and horrible. And yet, days later, Ognimir woke up with a spell in his head.
As some form of compensation-- he was offered a position of a Guardian. Slightly better pay and prestige than a dock worker, that was for sure. And over the years as a Guardian, he's actually come to like the job. Helping the people, doing the work. Though, on the harder days, he still wonders if it was some sort of set up-- where he was the target of some untested magic...