He was first mentioned by Elijah in the episode Klaus. He told Elena about the true nature of Klaus. Klaus is from another bloodline apart from the originals.
After Mikael found out that Esther had an affair with this man, he killed him and his entire family, which started the war between vampires and werewolves continuing to this day.
In Ordinary People, Elena and Rebekah discuss the origins of the vampires. Mikael confirms that he was the one who murdered Klaus's biological (werewolf) father, his family and half the village when he learned of Esther's affair.