Henry had seen the size of the Eldritch spider before it formed that nest to stay in, so he knew exactly what Mark would find in there!
But Mark had no idea what was in there, and he still entered it so easily!? That was like entering a cave in the mountains without knowing what you would see in there! Meeting the Elder Eldritch in that nest would be the equivalent of a human finding a hibernating bear in a cave!
Rumble!
Rumble!