A sentence containing a subordinate clause is a complex sentence. Although it was dark, and fear to spend the night in the jungle is a complex statement.
What are complex statements?
Complex statements are the sentences including the independent and the dependent clause. They are used in the sentences to express the complete and the incomplete thoughts.
The dependent clauses in the complex statements are:
Although it was dark, he still hadn’t turned the van’s lights on.
I was now worried that we wouldn’t arrive before nightfall, and my fears weren’t helped
Fearing that we might all have to spend a night in the jungle, I ventured another question beloved of children everywhere.
Therefore, the clause in the sentences makes them complex.