The Sixteen Realms

The Fourteen Realms, once referred to as the Sixteen Realms prior to the destruction of two of its worlds, are a variety of worlds parallel to Ninjago, which could be transferred via the Realm Crystal, an artifact that was located in the Tomb of the First Spinjitzu Master.

Following the apparent destruction of the Cursed Realm after the Preeminent was destroyed, along with the destruction of Djinjago, there are currently only fourteen realms. In the series, the realms act as the ultimate setting for the events that take place.

List of Realms
Don't put Ninjago on the list, as it does not count as one of the Sixteen Realms
 * Chima
 * Underworld
 * Cloud Kingdom
 * Realm of Madness
 * Cursed Realm (Destroyed due to immense flooding from the Endless Ocean)
 * Djinjago (Destroyed in consequence to the destruction of the Cursed Realm)
 * Departed Realm
 * Realm of the Oni and the Dragon
 * 8 other unknown realms.

Realm Passageways
Despite being of different worlds, the Sixteen Realms have multiple passageways and networks that can be used to access each other, with some forming naturally, and others having to be conjured.
 * Realm Crystal (All, except the Departed Realm)
 * Being overloaded by the Golden Weapons (From Ninjago to Realm of Madness; transformed into Mega Weapon by Garmadon, then melted by Nindroids to create the Golden Armor)
 * Traveler's Tea (From Ninjago to Realm of Madness, vice-versa)
 * Dragons (All, except the Departed Realm)
 * Blind Man's Eye (From Ninjago to Cloud Kingdom, vice-versa)
 * Book of Spells (From Ninjago to Cursed Realm, vice-versa; Destroyed by Lloyd)
 * Fire Temple (From Ninjago to Underworld, vice-versa; Destroyed by Flame)
 * Death (From Ninjago to Underworld/Cursed Realm/Departed Realm; Cursed Realm destroyed by Nya)
 * Mountain of Madness (From Realm of Madness to Ninjago)
 * Skulkin Vehicles (From Underworld to Ninjago, vice-versa)
 * Magic (All)
 * Allied Armor of Azure (All; Destroyed by Nya)
 * Unknown 'Back Door' (From Ninjago to Cloud Kingdom)
 * The mouth of The Preeminent (From Cursed Realm to current location, vice-versa; Destroyed by Nya)
 * Yin Blade (All; Destroyed by Cole)
 * Yang's Jar (From Departed Realm to Ninjago on the day of the Yin-Yang Eclipse; Destroyed by Cole)
 * Forever Rock (Chima Realm Passageway whose connection is currently unknown)
 * Resurrectea (From the Departed Realm to current location of the deceased person's body)
 * Being killed by one of the Departed Blades (From current location to the Departed Realm)
 * Killing someone with a Departed Blade (From the Departed Realm to location of murder)
 * Oni Masks (From Ninjago to presumably all other realms, including the Departed Realm)

Trivia

 * The concept of the Sixteen Realms is similar to the Nine Realms from the Marvel Comics. Notably, the fact that Chima is one of these worlds indicates that the Ninjago and Chima franchises share a universe, though whether worlds based on other LEGO properties are also presently unknown.
 * Consequently destroying a realm could destroy a sister realm within proximity.
 * The Sixteen Realms are surrounded by a infinitely vast, formless void, known as the Ethereal Divide.
 * It is a common misconception that Ninjago is part of the sixteen realms. In actuality, the sixteen realms are parallel to Ninjago. Including Ninjago, there were seventeen realms.