Miraak is a Ancient Nordic Dragon Priest who is also known as Fenrik when he was a priest of the dragon cult. the high ranking dragons of that time named him Miraak and gave him a dragon priest mask along with Vahl's ancestor Seldtis and named him Vahlok also known as Vahlok The Jailor. Later on, Vahlok would find a black book. Miraak would witness two dragons in immense battle one of the dragons would fall to the ground and Miraak would absorb it's soul. he would return to his study and in his power drunken state he would open the black book and then meet hermaeus mora they would strike a deal and turn against the dragons, that is all that is known so far it is rumored that Vahlok would clash blades with Miraak that's all we know. Also since he is a Dragonborn his common catchphrase is "And so the Last Dragonborn meets the first Dragonborn" meaning Vahl.
Neutral to Enemies to Friends
In the Dragonborn questline she was supposed to kill Miraak, Jordan didn't even have an idea about Miraak, until Miraak started stealing Vahl's Dragon Souls. Then Vahl wanted to tear his limbs off. When Miraak died, he returned to Solstheim, when he got back to Solstheim, he stayed at the The Retching Netch Corner Club the finally, Vahl went to the inn and recruited Miraak as a follower.
Miraak is believed to have a romantic relationship with Serana, hence both of them being immortals. It is also mentioned that he is somewhat related to Vahl, possibly her brother.