The World of IxalanIxalan draws much of its flavor from Mesoamerican civilizations--and their subjugation by foreign invaders. (In "Land of the Great River," below, an explicit link is even made between Ixalan and the adventure The Hidden Shrine of Tomoachan, which inspired it.) This should make it easy to introduce Ixalan as a new exotic continent in the World of Freeport; after all, the City of Adventure's history has much in common with Caribbean pirate havens.
One of the chief questions to answer is where to place Torrezon, the origin of the Legion of Dusk. The Hexworth Inquisition would attempt to rally all the nations of "the Continent" nearest Freeport against the rise of a nation openly ruled by vampires, so even the infamous Bone Lands would be an unlikely home. However, the azhar of Kizmir provide ample precedent for conquering armies appearing out of a previously unknown quarter to change the face of the known World of Freeport. Placing Torrezon on a new continent on the far side of Ixalan from Freeport seems to make the most sense--and would make Ixalan into a critical buffer state between the many nations of the Continent and the advance of the Legion's war machine.
Alternately, Freeport alone could be inserted into the plane of Ixalan, as part of (or even instead of) the Brazen Coalition.
Races of IxalanHumans have the same traits as given in the Player's Handbook, while Orcs have the same traits as PHB half-orcs.
Merfolk, vampires, and goblins previously appeared in Plane Shift: Zendikar, but Ixalan's versions are treated as separate races here. A sidebar provides guidelines for treating both settings' vampire races as subraces of the same species, and the same is done for goblins. Merfolk use the same traits as in Zendikar, but have two new Ixalan subraces: Green Merfolk and Blue Merfolk. As with Plane Shift: Zendikar, goblins and merfolk can easily be used with Freeport, even if the larger setting of Ixalan is not. (Vampires would have to be far more secretive to operate in Freeport.)
One entirely new race, the sirens, is introduced here as well. These avian humanoids have arms that serve as wings, and have a flying speed from 1st level. (This can easily circumvent many obstacles that low-level parties would find challenging, so this race should be allowed as PCs with caution.) Sirens typically inhabit island coasts, which could make them a potential hazard to Freeport's shipping traffic if they reach the Serpent's Teeth.
Land of the Great RiverThis section gives a map of Ixalan with brief notes about some of the locations shown on it. Following this are treasure tables for randomly generating art objects appropriate to the setting--and which will be useful in any Freeport campaign, whether Ixalan is used or not.
An Ixalan BestiaryMost of the creatures listed here can be represented by stat blocks from the Monster Manual or Volo's Guide to Monsters. Dinosaurs are among the most common beasts here, and this Plane Shift provides both alternate, in-world names for many species, as well as stat blocks for some truly epic saurians.
Other new monsters include the sunbird (phoenix) and chupacabra.
Appendix: The Colors of MagicThis section discusses the five colors of magic used in Magic: The Gathering, and how a "color alignment" can used a shorthand for the effects of spells or the personality traits of a creature. This material has no rules content, just role-playing hooks to include this central feature of the CCG in a D&D campaign.
This adventure is designed for a party of pregenerated 4th-level characters who belong to various factions within the Ixalan setting. If the vampire cleric Alante is kept as a PC or NPC, then the adventure could easily be adapted for other characters from a home game set in (or visiting) Ixalan.
X Marks the Spot: A Plane Shift: Ixalan Adventure
DMs who do not wish to run the adventure can still plunder it for ideas. The pregenerated characters would make excellent Ixalan NPCs, and Ellie and Turk would fit right into any Freeport campaign without any changes. In addition, Appendix D: Hidden Plunder presents six low-level magic items appropriate to both Ixalan and Freeport.
The influence of The Hidden Shrine of Tomoachan on Ixalan is apparent from the use of a temple of the bat god Aclazotz as an adventure site. DMs running that adventure (updated to Fifth Edition in Tales from the Yawning Portal) can use Plane Shift: Ixalan and X Marks the Spot to help flesh out the area around Tomoachan, and to provide hooks for why characters wish to seek it out.