They haven't explained it yet. The writing throughout the whole story has been very good so far.
We don't what his goal is or why or how he's done it. We just know that he is apparently trying to stop the Beyonders from eradicating the whole multiverse, and we don't know why the Beyonders are doing that. He's been working with Molecule Man, who is very powerful and has been traveling all through time and space I'm sure. He may have acquired some vast power source.
His cult of followers, the Black Swans, have said he's the cause of the incursions. But he told Dr. Strange that he created a religion around himself. He has his own creation myth apparently...some shyt about a universe instantly dying when Rabum Alal was born.
I think the Black Swans have never even seen him. The two of them that brought Dr. Strange to meet him had gouged out their own eyes to be in his presence because the light he radiates is unbearable or something like that lol.
It makes little sense now but I'm sure all will be explained soon. This whole story arc is over 3 years going and Doom was only a bit player until he was revealed as Rabum Alal.
They were all part of secret clique. Namor turned on Osborn and joined up with the X-Men so they had beef and Namor was no longer part of Osborn's Cabal. Doom and Namor had a business partnership that predated the Cabal so he didn't like that. He didn't respect Osborn's beef with Namor and felt he should still be at the table.