You determine the cost to cast Hour of Revelation before you pay any of that cost. For example, if the only nonland permanents on the battlefield are Tezzeret the Schemer and nine Etherium Cells he’s given you, Hour of Revelation costs {W}{W}{W} to cast. You may sacrifice three Etherium Cells to pay this cost, even though there are no longer ten or more nonland permanents after you produce {W}{W}{W}.

There are many important moments in the story, but the most crucial—called “story spotlights”—are shown on cards. These cards have the Planeswalker symbol in their text box; this symbol has no effect on gameplay. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.