Impregnable shrine, .... that you cannot kill despite killing EVERYTHING else. The 3/3 mission for Triple Threat campaign... I did it three times, killing everything in sight, only to be confronted with a lonely impregnable shrine that I cannot harm. Spent an hour googling how to get past it with no luck.
You just have to wait around the shrine for enemy heroes to be summoned and eventually you will get to the last one and when you kill it you win. I got frustrated at this as well after doing it a couple times but i saw people were completing it and just decided to keep killing champions as that is how it does say to win.
Thank you for the answer, I was also trying to figure out what was happening and googling the topic I found this thread so I kept destroying enemy champions until a moment eventually the enemy shrine explodes and you win.