Kaecilius was a former member of the Masters of the Mystic Arts who turned against them due his desire to become more powerful and believing that his master was preventing him from doing so by hiding things from him, a faction of mystics loyal to Kaecilius left the Masters alongside him.
When a dimensional rift was opened due to a conflict between the Hand, and the Avengers, Kaecilius used that rift to open a portal to the Dark Dimension. Kaecilius used that opportunity to harness the powers of that dimension for himself and his Zealots.
With his new powers Kaecilius decided to launch an attack on Doctor Strange's Sanctum Sanctorum. He allied himself with a tentacled creature from the Dark Dimension that helped him capture his former master master and teammates so they wouldn't interfere with his plans.
The Avengers were trying to close the dimensional rift, but weren't able to, Director Fury believed Doctor Strange would be able to close the rift so he sent a group of Avengers to contact him. When they arrived at the Sanctum Santorum they found Kaecilius, and the trapped mystics, they were able to free Doctor Strange who upon being freed, teleported his Sanctum Santorum, Kaecilius and the Dark Dimesnon portal to the Avengers Academy, so the Academy heroes could help free the rest of the mystic and defeat Kaecilius.
Kaecilius was able to summon the Dweller-in-Darkness to help him. He submitted to the Dweller and gave him his minions since he believed the Dweller was the master of the Dark Dimension he was looking for. The Dweller was defeated by the Avengers and Kaecilius found out he wasn't who he was actually looking.
Eventually Kaecilius fought the Avengers himself, but ended up defeated and incarcerated. His efforts weren't in vain though. The true master of the Dark Dimension, The Dread Dormammu, was able to cross over to their dimension thanks to Kaecilius being able to keep the portal open long enough.