What seal did Dean break? Although there are thousands of seals (as stated by Castiel), only 66 need to be broken for the devil to be released. Even though almost all of the seals were broken by demons, the first and the last were broken by Sam and Dean. The first seal was broken by Dean, whilst in hell.
What were the seals in supernatural? The 66 Seals are metaphorical locks on Lucifer’s Cage created by God representing events.
How did Sam break the last seal? The Final Seal: The blood sacrifice of the first demon will break the final seal. When Sam Winchester killed Lilith, the first demon created by Lucifer, her blood formed a portal which opened the Cage and freed Lucifer unto the earth.
Do Sam and Dean stop summoning Samhain? This draws the attention of the Winchesters and the angels as the raising of Samhain is one of the 66 Seals. Sam and Dean kill Don, but Tracy reveals herself and successfully summons Samhain, breaking the seal.