The goblin ran through one of the gates at the edge of the map (and way beyond, out of sight) and cannot be attacked, since the char cannot follow through the map border. Fissure cannot be completed.
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You donât have to kill everything on the map to complete the Fissure. I believe when you kill the Boss on a Fissure Level a new Portal will open to the next Level.
Youâve completed it when you open the Large Chest on the last Level.
That single goblin is a fissure âbossâ. You kill that gob, it leaves a chest, which triggers end-of-fissure. And, as we can see on the screenshot, that gob has escaped beyond the playerâs reach.