As the older brother it was always his responsibility to watch out for his siblings. That these siblings were ruthless, irresponsible, different in there own way and immoral made it even worse. After all they are the Mikaelsons.
With much patience he was able to get his siblings under control, at least most of them.
Niklaus Mikaelson, the second bastard son, a really difficult child.
But 300 years ago it got too much for him. The death of his family was too much. And the worst was that he couldn't do anything to safe them.
To get away from his siblings he gave the responsibility to Elijah, his younger brother.
He ran, further than ever. He broke the contact to all of his siblings, even Elijah.
But today, 300 years later, is the day, he has to come back.
The reason is simple: REVENGE!
The name of this Mikaelson,
The name of the oldest and most powerful Mikaelson, is:
MIECZYSLAW 'STILES' MIKAELSON