Monster Train

The Stygian Guard is unlocked by casting a cumulative total of 250 spells. While this can be achieved with any clan, using the Awoken as your starting clan will make it easier, as they have various 0-ember spells, some of which increase draw on the next turn so you can cast more spells.

Most ways to speed this up involve Sting, the aforementioned 0-ember spell that increases draw. Getting a Vinemother will give you a new Sting every turn, or you may find Preserved Thorns, which adds three Stings to your hand.