The Horseman of Famine is an interesting idea to think about, particularly since his/her weapon/symbol is a Weighing Scale which is popularly known as the Scales of Justice.
Lorewise, it would be kind of interesting to view this champion whose host is/was a Former Demacian or Solari who acts as a self-proclaimed warrior of Justice but brings nothing but cruelty, famine, and despair. Also, if you want to incorporate the idea of "Balance" from the scales, this champion can be one of the few/or the only Darkin who is in complete symbiosis with its host, being able to change forms at will similar to Nidalee or Elise or maybe Swain's old toggle ult.
In terms of gameplay, the Champion will have to be incredibly aggressive in lane being able to deprive enemies of farm (gold) and slowly dwindling down their Health while keeping his or her side in power. Perhaps this champion can fall under the subclass of a Controller - Disruptor Support.
As for a passive, it could be something similar to the militaristic use of a Scorched Earth Policy. Gameplaywise, this would possibly mean denying minions similar to DoTA2, but that would have no counterplay for the enemy.
An alternative could be a passive that is a DoT activated by ~~attacks/~~spells that scales with enemy Missing Health which, at max stacks, applies a status ailment similar to Zombify from Maplestory for X seconds. This status ailment will make the next instance of healing be lessened and instead deal a percentage of the value as true damage to the target.
To make this champion work with this passive, it will probably need a strong harass tool as well as a built in heal of some kind. Maybe an AoE heal or maybe something. Or it could be a point and click or delayed DoT tether ability similar to Karma's W or Swain's old Q that functions like Pain Split from Pokemon which slowly equalizes the current HP (via healing or damaging) between two targets within a certain percentage rate that scales with skill level. <--This skill can be used on an ally or enemy. In turn, this champion would scale really well if its HP is low especially if it is below the enemy's.