A cosmology is, basically, an introduction to the dimensions or planes of existence that are present as part of the overall milieu of a campaign. The Nine Kingdoms of Arn campaign does not utilize the standard D&D cosmology. For this campaign setting, there is no Great Wheel of Inner and Outer Planes, as described in previous editions of Dungeons and Dragons. Instead, the Nine Kingdoms of Arn has its own unique set of planes of existence.

The planes that make up the cosmology are as follows:

• The Plane Prime: This is the home plane of the World of Thaelan, where the Nine Kingdoms of Arn exists, and the vast majority of the campaign takes place.
• The Veil: This plane borders the Plane Prime and serves as the boundary separating the Plane Prime from both the Spirit World and the Shadowlands. Game-mechanically, it replaces the standard Ethereal plane of other campaigns.
• The Spirit World: Coexistent with the Plane Prime and the Shadowlands, the Spirit World exists as a positive reflection of the Plane Prime, and is coterminus with other planes of existence. Game-mechanically, it replaces the standard Astral plane of other campaigns, being the means to access the Manors Divine and the Vaults of Heaven.
• The Shadowlands: Coexistent with the Plane Prime and the Spirit World, the Shadowlands exists as a negative reflection of the Plane Prime, and is coterminus with other planes of existence. The Shadowlands is effectively the standard Plane of Shadow, and serves as the means to access the Infernus.
• The Elemental Plane: A single plane made up of the fundamental elemental types that is coterminus with the Spirit World and the Shadowlands. This single plane replaces the various Elemental Planes of other campaign settings.
• The Manors Divine: Coexistent with the Spirit World, the Manors Divine are the home planes created individually by the non-evilly aligned Divine Patrons of Thaelan. Despite the varied alignments of the Divine Patrons, the Manors Divine often correspond to other campaigns' concepts of the so-called "Upper Planes".
• The Vaults of Heaven: Coexistent with the Spirit World, the Vaults of Heaven is the home plane of the Sentinels, a divine race created by the Divine Patrons to combat the intrustion of rogue Demons on the Plane Prime.
• The Infernus: Coexistent with the Shadowlands, the Infernus is the home plane where the Demons are held imprisoned. The Infernus, in turn, is comprised of six demi-planes: The Pit of the Infernus, Vecheron, Verekna, Og, Thanis and Diannor. The inhabitants of the Infernus are bound in service to the Darker Gods by the terms of the Grand Compact.
• The Ten Thousand Hells: These smaller demi-planes are coterminus with the Shadowlands, and are often created by very powerful beings, including the evilly-aligned Divine Patrons and rogue Infernal Lords.
• The Floating Kingdoms: A number of smaller demi-planes that are coterminus with the Spirit World, smaller bubble worlds created by various means, typically by a non-Divine entity. The Floating Kingdoms are often not permanent (though some are quite long-lived.) Some are created as prisons or strongholds by powerful mages, others are dreamscapes that have taken form, and some exist for no easily discernable reason.