Here's an overview of these two titles, each of which is a standalone game for 1-4 players:
In your hands lies the destiny of one of history's great civilizations. Under constant threat of attack, you must conquer new lands, oversee dramatic scientific and cultural advances, and lead your people into the era of empire. Expand too rapidly, and unrest will bring your civilization to its knees; build up too slowly, however, and you might find yourself a mere footnote of history. As one of eight radically asymmetric civilizations, you will compete to become the most dominant empire the world has ever seen.
Imperium: Classics features the Carthaginian, Celt, Greek, Macedonian, Persian, Roman, Scythian, and Viking civilizations, while Imperium: Legends features the Arthurian, Atlantian, Egyptian, Mauryan, Minoan, Olmec, Qin, and Utopian civilizations.
Buckle might include the word "finally" in his quote because the game(s) has/have been in the works for a long time, with NSKN Games having attempted to Kickstart an earlier version of the design in late 2018 (link). Of Imperium, Osprey's Pete Ward said, "Due to the differences between what that game was offering and what Imperium: Classics and Imperium: Legends are offering, both I and the designers thought that two new pages for his game would be the best option."
Adds Turczi, "When I joined Nigel a few years ago to improve his civilisation deck-building game, little did I know we'd come out with 16 unique civilizations, each with their own solo bot, and more than 400 unique cards." The two titles each bear a May 20, 2021 release date.