OK let me get more specific
The story DID change for DMC3, but it still works
20 years before DMC1, Eva died and Vergil was"lost". IMO, he ran after the demon's that killed eva, and then began his mad quest for power.
He was lost to evil, the fact that he came back in DMC3 won't change that.
Anyways, I think Mundus had something to do with Eva's death, but I don't know how much. I'm suspecting he was able to order the demons to kill the family, without leaving his prison that Sparda sealed him in.
The demon world HAS been opened a lot. But there is a big difference in each one.
First, the Temen-ni-gru was the "Devil's Tower" and I believe it was the original way the armies of the Underworld were going to enter the human realm. They could already get through the Hellgate, but the Evil-worshippers, the human ones that is, built the Temen-ni-gru with the demons to provide another entry point. Or an exit point. These humans wanted to go to the demonic realm and become one with Evil. The Hellgate probably worked only one way, the Temen-ni-gru could go both. However, Sparda shut down both the hellgate and the temen-ni-gru. The Tower was opened first by Arkham, you know this. But on Mallet Island, there was another gate. The island's gate was made in preperation for Mundus' return. The demons that had gotten through by other means were inhabiting the castle. Its a big deal since there are tests and trials that the demons did not make. Perhaps the prophecy compelled the original castellans to prepare the way for Dante and challenge his strength so that it may grow. So the gate is not fully in the human plane, but instead is only in the mirror world. IT has not yet reached completion (which would mean it would be completely on this plane) the castellans probably sealed it away with the philosopher's stone and destroyed the only elixer "blue stone" in existance, so that the mirror realm may not be accessed. The philosopher's stone alone could not open the gateway though. most of the gateways require the amulets, for without them , the gates will not open.(save for the hellgates, since they only work one way. Though the Sparda sword might have satisfied that need if it indeed arose.
So that is why Mundus needed to lure Dante there. The creation of the Elixer "blue stone" needed to occur and at the very least Dante's amulet needed to be aquired. Dante played right into Mundus' hand and opened the gateway. Mundus expected to defeat Dante, but he lost. Only to recover and crash through the gate that Dante had left open.
You know the rest.
