The ending of Underworld: Evolution closes the story of the Corvinus brothers: Marcus and William are both dead, Michael has survived as a powerful hybrid and Selene can now stand in sunlight. The vampire and Lycan conflict itself is still unresolved.
What happened to Michael Corvin? Marcus impales him during the attack at Alexander's ship, and he seems dead, but he wakes at the prison in full hybrid form and kills William. In my opinion, that is the sequel's best turn, because it makes the quieter half of the couple the one who ends the original werewolf, while Selene handles Marcus with the helicopter rotor blades. The two kills mirror the two brothers: one hybrid claw for the first Lycan, one set of spinning blades for the first vampire elder to wake.
Why does the sunlight reveal matter? Alexander asks Selene to drink his blood, it gives her great strength and healing, and it leaves her immune to sunlight. That is the rule change of the film, and I think it is a smart one, because it hands the saga a hero who can walk out of the night world. It could be argued that this sets her apart from the covens that Underworld Awakening later shows being hunted by humans, where Selene wakes from a cryogenic tank into a world that has turned on both species.
How does the ending compare with the rest of the saga? The prequel, Underworld Rise of the Lycans, is a contained 92 minute tragedy about Lucian and Sonja, and it closes its own story. Evolution runs 106 minutes and deliberately leaves a door open: Selene and Michael are survivors of both bloodlines, with no victory over either species. That is a franchise ending rather than a finale, and the story does carry on in Awakening and Underworld Blood Wars.
There is also a production angle. Evolution had a $45 million budget according to The Numbers, a clear step up from the first film, and the finale is where it shows: a helicopter set piece, a ship destroyed by Alexander and a full hybrid creature, rather than the gunfights and corridors of the opening act. Len Wiseman returned as director, so the sequel keeps the look of the 2003 original while giving the family history more weight. If you want to see where the story goes next, the Underworld timeline lays out the order.