season 3

Early on season 3 they have faced the magnificent seven (7 dangerous demon) who escaped from hell 

thruogh the gates of hell. on season 2 finale.They also introduced to a new character, a young women named  Ruby who uses a demon killing knife.

Ruby holding demon killing knife

Ruby is a demon that shows particular interest in Sam. At her urgings ,Sam discovers that all the killing that the yellow eye demon has some connection with him.Ruby won't discuss what it is she wants, but says that not all demons are the same or have the same motivations. She also says if Sam helps her she can save Dean. She also assisted Bobby in successfully rebuilding and restoring power of the colt. At the end Ruby tells Dean that most demons were once human, but they forget what it's like to be a human . For same reason she remembers, which is one of the reason that she helps the brothers. Dean and Sam came to know that Ruby was once human too, she sold her soul to a demon.  

Ruby kill one of the magnificient seven

 In the episode"bad day at black rock", they found that John(father) had a secret starage room at black rock.Dean and Sam find that an object from the room is missing and set out to find it. The boys track down the burglars and get to know that the missing object was a rabbit foot. The rabbit's foot brings the boys to a series of lucky events until it is stolen by Bela , a thief and merchant out to use the supernatural world as a paycheck, and Sam's luck turns really bad.

Sam and Dean got really lucky (Rabbit's foot)

 Described by Bobby Singer as pretty damn far from hunter , Bela is an attractive Englishwoman who acquires magical objects for wealthy clients. Bela claims that she will steal anything to anyone for the right price.Dean threatens to kill Bela numerous times, but she stll continuously tricks and pranks the winchesters brothers.

The rabbit's foot

Bela

Sam and Dean argue over whether using the newly rebuilt Colt will help give them the upper hand in breaking Dean's deal with the crossroads demon who revived Sam. At the end the episode sam takes the Colt and while Dean is asleep, visits the demon and threatens her . She tells Sam , however that killng her will not break the deal since her boss holds the contract. Sam shoots and kills the demon anyway, along with the women it was possessing.
Then Bela was able to steal the Colt from the brothers . The following episode the trickster came and made fun out of Sam and Dean . the trickster sent off with a massage told Sam stop trying to save Dean from dying.
Sam's replacement chosen to appa rently lead the army of demons was revealed to be named Lilith, a white eyed demon who has possessed a little girl.The brothers eventually learn that Lilith is the demon holding Dean's deal. The brothers along with Ruby track Lilith down and attempt to kill her . Lilith was unable to stop Sam due to his mysterious abilities , however Dean's contract expires and his soul is taken.

Lilith(white eye demon)

Dean died attacked by hellhound(demon dog)