Plot and ending spoilers for Divine Divinity and Beyond Divinity follow...
Divine Divinity starts with the Black Ring capturing an angel and starting a ritual to destroy her. She splits into 3 to escape and enters 3 humans, who become 'marked ones' (the player's character, one who gets himself killed because he is stupid, and one who gets himself killed because he is gullible and stupid). After this the character awakes in the small healing village of Aleroth, and can do some quests and looting before exploring the catacombs there or leaving.
Zandalor shows up to save the main character shortly after you leave Aleroth, provides some background information, and takes off to do some investigating. You later meet up with him, and after he provides some additional info, you have more main plot things to do (gather 7 council members together to perform a ceremony to become the Divine One). He sticks around after that to provide advice and support, but doesn't do an awful lot (the usual 400 year old wizard concentrating on big picture stuff while the inexperienced main character does most of the work). The game ends with the hero defeating the main Black Ring members and the Demon of Lies, but arriving too late to prevent them from summoning the Lord of Chaos into a baby (Damian).
Unwilling to kill a baby, the hero (Divine One) takes him to raise himself, with Zandalor's help, which doesn't turn out very well. That is covered in the BD novella, included in the fansite kit
(17.7MB) with a summary here
, and in Beyond Divinity. The fansite kit also includes the background story to DD, covering the previous war with the Lord of Chaos.
In Beyond Divinity, the main character is a follower of the Divine One (who setup a group of Paladins to hunt down the remains of the Black Ring), who was pulled into another realm (Nemesis) by a demon when trying to take down a necromancer. After getting soul-forged with a deathknight, the two escape prison and try to find a way to get back to Rivellon to undo the soul forge. As this happened to be where Damian was banished, along the way they see the aftereffects of his actions, and learn more about what happened up to that point. Of course Damian shows up at the end and manages to break his banishment (which is where D2:ED and its backstory take over the plotline). I doubt the details of Nemesis are directly relevant to D2:ED's plot, so suffice it to say that as Damian attempted to get back to Rivellon, he became increasingly frustrated, angry, cruel and obsessed. The main connection to D2:ED's plot (at least based on the intro/trailers) is with the novella events (summary link above).