Stállu in Sápmi folklore is a giant human-like creature who eats people. It's a clumsy and dumb beast who can easily be outwitted by humans. It's awfully fond of challenges that test its strength.
This might be an actual scene from an upcoming chapter. Just how the boys are gonna outwit him exactly, you'll have to wait to find out!
The word stállu or staalo can also be used to describe a troll or a giant. In Tistow however, he shouldn't be confused with his cousins made out of stone.