Diana Von Jarl. The Canoness of the Battle Sister Order known as the Lunar Viel. Formerly a member of the Argent Shroud, she was raised to the rank of Canoness to lead her own order.
At first she was hesitant and fearful for the lives she would be responsible for. Which lead to her greatest sin. in her first Crusade mission, she was to scout out the Nurgle corruption that took place on a former Hive World. Unlucky to come across the Death Guard's own Typhus, The Wanderer.
Many of her sister's died due to her inexperience to lead. But through her will she brought victory for the Emperor and the Imperium by charging at a Plague Marine cleaving a fierce wound with her power sword. In the battle however she herself received a plague-ridden wound. One that left her scarred. A reminder of her gravest sin.
She would cover this sign of corruption with a silver mask. Painfully it would keep the wound from festering and adding to Nurgle's corruption of her body. And the pain would further inspire her to lead her order to hunt for corruption and heresy where night is darkest.

Diana Von Jarl is a creation of my named warlord for my Adepta Sororitas (Battle Sister) Army for the #warhammer40k tabletop game. This was the lore I came up with for her order and the first battle she took part in, which was against a Death Guard Army, led by Typhus.
Her design inspiration (which came out pretty convenient) came a little bit from Baldwin IV (aka The Leper King) of the Kingdom of Jerusalem. I'm not a fan of Crusaders, but the connection was perfect. To give the aura of someone struck with a horrible illness but still inspires their warriors on the field of battle, and s a force to be reckoned with.