3a) Standing within sight of the lower-left (Southwest?) corner of the realm, I used about 15 Pandemonium Tokens. One of them summoned more packs of enemies too the realm (as Tokens sometimes do), including one (Pack 1) 2-3 tiles from the realm’s Edge and another (Pack 2) hidden in Fog around a corner a similar distance away from me.
3b) As I moved to engage Pack 1 (which was, for some reason, moving closer to the Edge instead of towards me), Pack 2 came out of hiding and attacked, running into my Creature Trail in the same “turn” that Pack 1 reached the Edge.
3c) After defeating the creatures of Pack 2, I returned to the overworld to discover that Pack 1 was now standing on the Edge between the realm and the Infinite Black Whatever beyond.
3d) Not thinking of the possible consequences, I took another step towards Pack 1’s precarious position, giving it the opportunity to step out into the IBW and out of sight. Luckily this meant it no longer counted for the “slay all enemy packs” duty, which was active at the time. I’m going to assume it de-spawned upon making its escape.
Playing on Windows
If it helps, I had just recently finished grinding through a great many Major and Minor Sigils (more than 50, but less than 100).