Scrolls is free on Android devices. Does it mean I have to buy it again

Poison is used by a variety of scrolls in Decay and a few in Growth. All scrolls discussed are Decay unless otherwise stated.

Poison is an effect which can be applied to creatures (not structures). (An effect is a modifier attached to a unit which doesn't count as an enchantment.) It may be applied in several ways, but always has the same result. Quoting from the in-game description on a poisoned unit:

Unit takes 1 damage at the beginning of owner's turn.

• A unit becoming poisoned does not deal any immediate damage.
• If a unit is poisoned twice (e.g. by applying Ranger's Bane twice), they count separately and will deal 1+1 = 2 damage when they trigger.
• When the unit is healed, this removes all poison.

Units may become poisoned in several ways; directly by use of Ranger's Bane (Growth) or Brain Lice (note that Brain Lice applies poison and separately its own enchantment for the draw effect), or indirectly by the abilities of another unit (Vitriol Aura (Growth), Bitter Root, Blightbearer, Pest Dissimulator).

Poison damage is a type of non-combat damage. It is increased by Curse enchantments and ignores armor.

Blightbearer and Infectious Blight are scrolls which receive or deal poison damage but do not use the Poison effect itself; notably, Infectious Blight acts very much like poison but does not display the graphic for poisoned units.

Miasma Well is a structure which does damage to poisoned units only.

The game will be available on every device connected with the same google account, if you try to buy it twice it will say "you already own this item". However you must reinstall the game any other devices where you didn't purchase the game. The game run well on phones too you just have to copy the apk file, the full version will unlock itself on the first startup.