Fairies

Fairies are mischievous supernatural creatures that come in a variety of forms and can only be seen by those who have been to the fairy realm or those they allow to see them. When summoned by a spell, they can make deals with humans, oftentimes exchanging their services for a price, such as the right to all of the firstborn sons in an area. Fairies also appear to have great power, and will remain in an area until banished by a spell.

Weaknesses
Iron

Iron is proved to be harmful against fairies, but doesn't kill them.

Silver
Silver, like most other supernatural creatures, is also something fairies do not fare well with.

Sugar/Salt
If someone pours salt or sugar in front of them, they have to stop to count each grain one by one.

Cream
They also love cream, which, like alcohol, intoxicates them.