Sombra isn't really up to reform, but The Lord of Chaos feels the need to teach him how to be 'nice.' Since Discord is totally the sub-princess of friendship, he's best to 'train' Sombra. What could possibly go wrong?
A lot of things.
Cover art source: https://www.deviantart.com/assassin-or-shadow/art/Discord-Reforms-Sombra-449733671