Dormammu and Umar are able to share each others power due to a shift in the cosmic axis as Dread 1 indicates here:
With this imbalance Dormammu is able to enter the essence or manifestation of Eternity in order to remake the universe in his own image. With the Amulet he enters Eternity's dimension and announces his plans (neglecting Umar in the process) and enters what he calls "the blood of Eternity".
He didn't get to do much in terms of universal scale manipulation due to his selfishness towards Umar who as a result, foiled his plans.