The most niche of the commons in the faction, this is the card that should hit the discard pile the most when in need of magic. It still can be used as a Faithful Servant when needed of course, and in all the many roles that a 0c 2/2 melee can serve, but the main role is to put pressure on the enemy summoner, something the deck needs. In combination with the Epic Event Convert, you can kill a common enemy unit near to their summoner, Convert the unit into a Doom Cultist, and then attack with this Doom Cultist and be an added threat just based on proximity.
Another use with the event Death Rite instead of Convert, is to attack with the Doom Cultist the unit protecting the enemy summoner (assuming it has 2 or less life), and after killing it, sacrificing the Doom Cultist with Death Rite, opening the way for a ranged unit to attack the enemy summoner and meanwhile causing damage with Curse.
Note that for Convert you don’t need the Doom Cultist in-hand, so generally this is the safest unit to discard for magic.