Because
The Reason Is
conjunction
When someone asks you 何故, or "why," you did something, you respond with, "何故なら blah blah blah." That makes it mean because. Why did you do that? Because I was mad! Why did you dance? Because I was happy!
This reading is an exception but you learned them already because they're the same as the word 何故, which you learned a few levels ago. It just has some kana attached to it!
何故なら、彼は魔法使いだったのです。
Because, he was a wizard!