"Give your army the upper hand against enemy troops with this deadly poison! Defending units that linger in a Poison Spell's toxic cloud will move slower, attack slower and take damage with increasing severity over time. Poison Spells do not affect structures."
Summary
The Poison Spell is a spell added in the July 1, 2015 update.
It does damage and slows down all enemy Clan CastleTroops, Heroes, and Skeletons inside its area of effect, however buildings remain unaffected.
Offensive Strategy
A Poison Spell only takes one housing space, whereas a Lightning Spell, the other spell that can be used to deal with Clan Castletroops, currently takes up two housing spaces. This makes the Poison Spell very appealing as a way to deal with Clan Castletroops along with a few Troops.
A Poison Spell has a circular area of effect for scaling damage, whereas a Lightning Spell has individual lightning strikes in a predetermined pattern, so both can be used in a similar fashion. Which is ultimately more effective depends on the defending base's layout, as well as the what and where the Clan Castletroops are, and the player's army composition.
As of September 18, 2015, Poison Spells now do scaling damage in a similar manner to a single target Inferno Tower, just to a lesser extent. This scaling effect dramatically affects high hitpoint units, but does not deal the same scaling damage against Heroes, instead dealing little appreciable damage.
A Poison Spell can take a significant amount of time to kill troops, depending on the type of Troops inside the Clan Castle, so make sure that you have enough time left to finish your attack after the Clan Castle troops are dealt with.
A level 1 Poison Spell can easily kill all the first few troops unlocked in the Barracks, as well as any level Wizards, Witches, Skeletons, and Minions. Upgrading this spell, however, is still useful, as higher level Poison Spells kill them faster and leaves more time to finish the attack.
If you see a Dragon, Balloons, or Valkyries on defense, it's good to use a Poison Spell on them, as the scaling effect is deadly against high hitpoint units.
Although a single poison spell will not kill out very high hitpoint troops like Dragons and P.E.K.K.As outright, its slowing effect makes them highly vulnerable to other troops while active.
Multiple Poison Spells stacked will speed up the scaling process, but will not increase the maximum damage per second.
While not a popular strategy, Poison Spells can be used to stack damage, making it possible to kill any Clan Castle units with enough spells, however.
As of the March 2016 update, defending troops now run away from the area of a Poison Spell, giving the defender an advantage. However, if the attacker places down troops in the vicinity, they will prioritize engaging the troops rather than escaping the poison.
A good offensive strategy is to lure the Clan Castle troops to the corner and then attack them with high damaging troops like the Wizard or a high number of Archers. Usually, when the Poison spell is dropped on any defending troops, and if they don't have any target, then the defending troops will walk out of the radius. So, it's good to attack them with troops while the defending troops are inside the Poison Spell, to get the full potential of the Poison spell.
Trivia
Prior to the Poison Spell, strategies that did not include a Clan Castle lure utilized a well-placed Lightning Spell to destroy defensive Clan Castle troops. With the introduction of the Poison Spell, players now have a viable alternative to destroying Clan Castle troops.
The Poison Spell used to deal static damage to enemy units, making them more effective at killing Skeleton Traps, in addition to Clan Castle units. Since the Poison Spell changed to progressive damage, this is now a less than favored approach, as they tend to slip out of the spell's circumference before being destroyed.
Ghosts emerge after a Poison Spell is thrown and they keep emerging for 3 seconds along with a creepy laughing sound.
Poison Spell's brewing time has been reduced multiple times, from 15 minutes to 3 minutes.
