I have searched and searched, but have been unable to find a description of the "poison" mechanic in Mojang Scrolls. I would appreciate an explanation.
Poison is an ability that is currently only found in two growth enchantments: Vitriol Aura and Ranger's Bane, although as of writing the Decay deck is currently going through testing, and according to the Scrolls database will contain at least 8 other scrolls that utilise Poison.
The ability damages the target creature by 1 hit point at the start of each round for a specified number of rounds.
It's a bit hard to talk about scrolls that are currently in development, but as an aside I notice that Blightbearer will take "poison damage" before it attacks, which is essentially not how the "poison" keyword is usually used. I would hazard a guess that this is simply categorising the damage type, enabling scrolls such as Miasma Well to modify the effect.