The entire map will be filled with purple glyphs and Margwas. If you touch the purple glyphs you will become damaged. Additionally, your screen will turn red every thirty seconds. When this happens, you must run to one of the white orbs floating around the map to prevent yourself from going down.
You may notice the stone goblets around the map will have red flames instead of blue flames, preventing you from becoming the beast. To recharge a stone basin you must kill at least one Margwa. All four players will need to be in Beast Mode, three of them in Canals, Footlight, and Waterfront, with the final player being in the center of the map near the crane.
Beast mode will last for an infinite amount of time. However, if you revert to human form, let the Margwas hit you too many times, or allow the screen to turn red for long enough without touching a green orb, you will need to kill another Margwa to recharge another stone basin. So always be ready to grab a white orb as you perform the next step.