Explain why the chapter is called "The Evil Spell."

In the book, Bridge to Terabithia, chapter nine is called “The Evil Spell” because it is raining so much, Leslie thinks an “evil being” has cast a spell on their make-believe kingdom. This chapter also comes at the heels of a prior chapter that discussed Easter and the crucifixion of Christ. Since it is evident that Leslie is not familiar with Bible passages, church, or religion, it seems interesting that in chapter nine, she would appear to demonstrate a spiritual side in their pretend realm Terabithia:

"Let us go even up into the sacred grove and inquire of the Spirits what this evil might be and how we must combat it. For of a truth I perceive that this is no ordinary rain that is falling upon our kingdom."
"Right, queen," Jess mumbled.

It rains so much after Easter that the creek Jess and Leslie use to cross into Terabithia and their shack-pretend kingdom there has risen. They both use a rope tied to an apple tree to swing across the creek into their pretend territory. Water comes through the top of their shack and pretend castle, which puts a damper on their plans to play the role of king and queen of Terabithia. This is what chapter nine and the evil spell on their pretend castle is all about.

The simplest and most direct answer for why this chapter is called "The Evil Spell" is because Leslie basically says the exact same thing.

Methinks some evil being has put a curse on our beloved kingdom.

The reason Leslie says that is because the weather is just terrible throughout this entire chapter. It rains and rains, and the creek crossing to Terabithia gets more and more harrowing. Throughout the entire chapter, Jess has feelings of impending doom that get worse as the chapter progresses. Leslie picks up on his feelings, and she can see the creek getting more fraught with danger. Both children know that the danger is increasing, and both children wish the rain would stop; however, the rain just keeps coming. It's as if the world has been put under some kind of evil curse or spell. "The Evil Spell" seems to be a very appropriate title for this chapter.