During the introduction video, you can just barely make out Overlord GAIA's Face◊ for the split second it shows. However, getting a good glance at it makes you realize how evil he really looks, even sporting a permanent Slasher Smile.
GAIA transforming Crimson into NeoCrimson, brainwashing him and forcing him to take on the aspects of all the Chaos Generals and the Chaos Lord at once.
The Chaos Tower, where you encounter Overlord GAIA, goes out of its way to remind you that is The Very Definitely Final Dungeon with its creepy, bluish floors, and the equally chaotic and nightmarish music that plays throughout. Not to mention that there are maximum-level traps and dangerously high levelled Ultimate and Mega Digimon everywhere.
And then there's the fight against Overlord GAIA himself. It starts out somewhat easy, as a battle against a single, but surprisingly strong opponent... but then his arms come out of his body and the battle takes a turn for the worse.