On one of the blogs I follow, a reader asked about whether she should use omniscient viewpoint in her first chapter. In omniscient viewpoint you read the thoughts of all or most of the characters' heads as well as get an overview of what is happening outside of the character viewpoint. Here is my reply.
These days, people have so many ways to entertain themselves that reading has a hard time even being considered, let alone being in the top five of how someone wants to spend their down time and money. As writers, we have to figure out why our stories should be part of that down time. What does reading this story give that no other entertainment gives?
Science has helped here by studying the brain activity of readers. "Being there" is what readers want. They want to be in the story and feel, see, taste and sense through character viewpoint. The character's reactions make our brains experience the same thing, and this makes the brain and reader very happy. Omniscient viewpoint's main target is the intellect which works for some readers, but it doesn't offer those wonderful I'm-there brain reactions a majority want.
Another reason not to use omniscient is that newer writers suck at it, every single time. It's a landscape of bad craft land mines that the ignorant writer skips happily through, blissfully unaware that the reader experience is being destroyed in his path. Once the craft is master level after years of writing and the writer reaches excellence, he may decide omniscient POV should be tried. Even then, he'll probably start a few chapters then go "Hell, no. This is tripe." End of omniscient experience.
As a writing teacher, I'm guessing that the real reason the question asker wants that fantasy first chapter as omniscient is she wants to info dump all her worldbuilding in one spot because she thinks the reader needs this pile of worldbuilding vomit NOW. Spoiler alert: They don't.
So, listen to agents, editors, other authors, writing teachers, and readers, and just so no to omniscient.