The Definitive Edition is the updated re-release of the game. The Windows version also includes the original release, now called Classic (unless you want the extra achievements, or to try a mod or standalone campaign that wasn't updated for the Definitive Edition, there is not much reason to play the original release).
Any copy of D:OS 2 will be the Definitive Edition.

The Divine Edition is the Definitive Edition of the game, plus the 'Divine Ascension' DLC content (soundtrack, a pdf map, art and lore books).

The Eternal Edition is a package consisting of the D:OS 2 Divine Edition, D:OS Enhanced Edition (not Classic) with its Collector's Edition extra content (soundtrack, design documents, art pack) and the earlier three games Divine Divinity, Beyond Divinity and Divinity 2 Developer's Cut.
If you don't want the earlier games, you don't need the Eternal Edition.
If you do want the earlier games, and specifically want the original release of D:OS (it is 32 bit and uses DirectX 9; the EE became 64 bit and requires hardware support for Direct 11), it would be better to get the D:OS 2 Divine Edition and D:OS EE Collector's Edition separately; that would include all the same extras, though cost slightly more, as the D:OS EE CE is a 2 pack).


The Sir Lora DLC is not included with the game or in the Divine or Eternal editions (only being added with the Definitive Edition release). It was added for free to everyone who bought D:OS 2 before the DE release, but after it must be purchased separately.
As detailed above, the Divine Ascension DLC upgrades the standard game to the Divine Edition, and does not contain any in-game content.