Poison is a skill that allows a unit to inflict additional damage on another unit (or more rarely, group of unit) at the start of every turn.
Commonly found on Nature and Chaos races, this effect has a continuous, passive nature (meaning that once the target recieves it, it doesn't require for the attacker to stay alive) and stacks additively with attacks (meaning that further attacks will increase the amount of damage inflicted). It can affect all enemies, except for warlord.
Trixy, the Spider Queen can inflict poison on all enemies with her first skill.
Poison units can be classified in Poisoners and Poison granters, depending on which unit is the target of the effect.
Poisoners
These units will inflict the skill directly to an opponent, usually when attacking.
| Name | Tier | Race | Description |
|---|---|---|---|
| Mummy | 
|
Chaos | Poisons attacked enemy |
| Wyvern | 
|
Nature | Poisons all enemies in line with every attack |
| Anak, Princess Dragon | 
|
Nature | Poisons all ranged enemies with every attack |
| Rock |
|
Nature | Poisons random enemy in line when entering the field |
| Stone Guardian |
|
Nature | Poisons enemy when Attacked + grants Poison to ranged allies at every Start of Turn |
| Embalmed Priest |
|
Chaos | Poisons all enemies when killed + grants Poison to random ally at every Start of Turn |
| Undead |
|
Chaos | Poisons enemy when Attacked |
| Leader Nilen |
|
Chaos | Poisons all enemies that enter the board + Summons Hollows with Poison at every enemy death |
| Marquis de Sat |
|
Nature | Poisons every enemy who attacks |
| Hollow | 
|
Chaos | Poisons attacked enemy |
| Melia, Forest's Daughter |
|
Nature | Poisons 2 random enemies at every End of Turn |
| Lizardfolk |
|
Nature | (Poisons attacked enemy) |
| Poison Flos |
|
Nature | Poisons all enemies with every attack |
| Zombie |
|
Chaos | Poisons attacking enemy when Attacked |
| Coba, Temple Keeper |
|
Nature | Poisons adjacent enemies at every End of Turn |
| Planty |
|
Nature | Poisons attacked enemy |
| Plant |
|
Nature | Poisons attacked enemy |
| Trixy, the Spider Queen | 
|
Chaos | Poisons all enemies on the field |
Poison granters
These units will grant other units the skill, which will then be used on enemies.
| Name | Tier | Race | Description |
|---|---|---|---|
| Embalmed Priest |
|
Chaos | Grants Poison to random ally at every Start of Turn + Poisons all enemies when killed |
| Leader Nilen |
|
Chaos | Summons Hollows with Poison at every enemy death + Poisons all enemies that enter the board |
| Stone Guardian |
|
Nature | Grants Poison to ranged allies at every Start of Turn + Poisons enemy when Attacked |
| Argios |
|
Nature | Grants Poison to adjacent allies when entering the field |
| Lady Wild |
|
Nature | Grants Poison to random ally at each End of Turn |
| Leaf Child |
|
Nature | (Grants Poison to random ally on Death) |
Comparison with other skills
Poison vs. Vampire
Both these skills have the effect of increasing the amount of damage inflicted to enemies (warlord excluded).
Differently from Vampire, Poison can be found outside of the Chaos race, it stacks additively (meaning that further attacks will increase the amount of damage) and once inflicted it doesn't even require for the attacker to attack every time.
On the other hand, it doesn't help the attacker to recover some of the lost HP and even in the case it'll be enough to kill the unit it procs after other skills, giving the attacked unit the time to use new-turn effects (like Heal or Attack boosts).
Poison vs. +ATK (Attack Buff)
Both these skills have the effect of increasing the amount of damage inflicted to enemies.
The main difference is a bit technical, and given by the time they act: Poison is a permanent debuff that affects a defending unit during the pre-attack phase of the following turn, and as such it will require a bit of time to show its effects (making the unit able to use pre-attack skills in said timelapse, even if Poison would kill it).On the other hand, +ATK is either a temporary or a permanent buff that affects the attacker during the attack phase of the turn, and produces a direct effect in said turn.
A second difference is the target of the effect: once inflicted, Poison stacks on the defending unit (so it doesn't require the presence of an attacking unit anymore), while +ATK stacks on the attacking unit.
Furthermore, Poison doesn't have effect on warlord, while +ATK does.
Poison vs. Bleed
Both of these skills apply debuffs that cause targets to take additional damage over time, the main difference being when that damage occurs: Poison takes effect at the start of the affected unit's turn, while Bleed damage is applied immediately after the afflicted unit's attack. This makes Poison the faster and more reliable effect, as Bleed may not even take effect if the afflicted unit is unable to attack nor prevent the attack damage. On the other hand, Bleed damage can apply multiple times per turn if the afflicted unit has extra attacks.
Neither of these two can be applied to a warlord. Some poison effects exclude buildings as possible targets and while bleed can be applied to buildings, it has no practical effect since buildings are unable to attack directly.
Note about Poison effect
It's worth noting that "Poison" is actually implemented as the ability to inflict the Poison effect, which will then stick to the enemy unit causing its HP to decrease over time - and as such, when silenced they can lose such deprecative skill.
Therefore, units named here as "Poisoners" are picked by the in-game filter as "Skill granters", rather than Poisoners themselves. Similarly, those named here as "Poison granters" aren't picked by the Poison filter unless they are also "Poisoners" themselves (but usually picked by the "Skill granters" filter). Albeit this would technically the way to go, this classification won't provide enough usable infos, and is mentioned here only to explain some possible discrepancies when filtering units.