What Makes Idigam Unspeakably Alien in Practice?

From time to time in discussions of the setting, the Idigam are mentioned as these eldritch, unspeakably alien entities from another time. The summary about them says as much, but doesn’t go into details – just describes them in abstract terms.

But are there any actual, canonical examples of Idigam known in more detail than a mere mention? What are they like? What concrete, actual properties thereof make them so alien? Are there any canonical examples of these entities with specific traits, abilities or behaviours that have resulted in them being described like that, and if yes, what are they and what are they like?

This question is not about power level, but about their unique qualitative differences that make them hard to comprehend (both to characters and to players) – whether universal ones, or ones specific to any named examples or subsets thereof.