"Nothing takes down enemy troops better than this deadly poison! Cast this spell to create a lingering toxic cloud that slows and rapidly drains the health of enemy units. Poison Spells have no effect on enemy structures."

Summary

 The Poison Spell is a spell added in the July 1, 2015 update. It is the first spell available in the Dark Spell Factory. It does damage to all enemy Troops, Heroes, and Skeletons inside its area of effect, as well as slow them down, but does not damage buildings whatsoever.

 Offensive Strategy

 A Poison Spell only takes one housing space, whereas a Lightning Spell, the other spell used to deal with Clan Castle troops, currently takes up two housing spaces. This makes the Poison Spell very appealing as a way to deal with Clan Castle troops. A Poison Spell has a circular area of effect for progressive 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 style of attack and defending base's layout, as well as the Clan Castle troops, themselves. A Poison Spell deals its damage progressively, so make sure that you have enough time left to finish your attack after the clan castle troops are dealt with. Damage done by a Poison Spell stacks, so placing multiple spells at the same time will do additional damage with no damage penalty. A level 1 Poison Spell can easily kill all Tier 1 troops as well as any level Wizards, Witches and Minions.

 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. 7 level one Poison Spells are enough to take down a level five Barbarian King.