Legality depends on the tournament format -- The Terror of Serpent's Pass can be legal in some formats and banned or restricted in others
The highlights of this deck include Rumbleweed, a monstrous creature that comes loaded with an enter the battlefield Overrun effect, an Ancient Greenwarden, which has an incredible ability to copy your land-based triggers, and Embrace the Unknown, which can help turn your lands into Impulse-draws later in the game
The larger point, and one that many players expressed in the comments below this post, was that such crossovers existing in the first place is problematic
With cards like Scute Swarm, Field of the Dead, and Springheart Nantuko, this gets game ending incredibly quickly, so much so that people are already brewing cEDH decks around it