The role and powers of the champions

The champions of the world-isles of Braythe are among the most powerful individuals in this reality. Usually, they have been people of considerable might even before being chosen - the prerequiste to becoming a champion is being level 20 (or having 20 hit dice). Still, they are no gods, and have their clear limits. Many champions are humanoids, but a few world-islands have been created by dragons or other supernatural creatures.
Most importantly, while the champions do not age anymore, they are in no way immortal. A champion can be killed. And as the life-essence of this chosen character and that of their world-isle are deeply tied to each other, the death of a champion means the death of the world - and vice versa.
The champions have a variety of titles on different worlds - guardians, protectors, defenders, and so on. The most common one aside from champion is paragon. Those champions that decide to rule their world often take on titles that show their power and status, like king, queen or emperor.

The champion powers

The granted magic to create the new world seems to be a one-time thing: After the world-island is finished and transported to Braythe, the champion cannot change or rearrange the land anymore. Their influence also does not extend to the void itself: While some champions learn spells or other ways to influence the purple nebula, their champion powers do not grant them dominance over the void.
Aside from being at least level 20, every Braythian Paragon gains the following set of powers and abilities:

• Immunities: All champions are immune to aging, disease, poison and any kind of mind-affecting spells.
• Danger sense: Champions have a special connection to their world, making them sensitive to danger. That does not make them clairvoyant, but they do get a feeling of where they should take a closer look (or send someone there). This ability works even if a champion leaves his or her world.
• Increased life force: The champions tap into the life force of their world-island, quadrupling their hit points. This includes future increases after becoming a champion, but no temporary increases from spells or spell-like effects.
• Increased speed: The speed of a champion doubles.
• Multiattack: A champion attacks four times every round. If the champion had more than one attack per round before becoming a champion, the extra attacks stack with the four attacks from the multiattack feature.

Legendary Resistance

Legendary Resistance (3/day): If a champion fails a saving throw, they can choose to succeed instead.

Legendary actions

Every champion can take three legendary actions, each one resembling a spell of a certain level. The spells have to use the champion's world-island itself or its inhabitants, but are not limited to the spells of a specific class or school of magic. For example, a champion might be able to summon helpful creatures, or create earthquakes and floods. Non-specific spells like wish are excluded, as well as any spell that use something from outside the world-isle, like meteor swarm or summoning creatures from other planes. Only one legendary ability can be used at a time and only at the end of another creature's turn. Braythian champions regain spent legendary actions at the start of their turn. The three legendary actions are set when the character transforms into a Braythian champion, and don't change later.

• 7th level spell: The champion can cast one 7th-level spell at will.
• 8th level spell: The champion can cast one 8th-level spell at will.
• 9th level spell: The champion can cast one 9th-level spell at will.

Lair actions

Every Braythian champion has special powers tied to his or her world. In that context, their world-island is considered the champion's lair. This extends to the first two chain-links of the attached world-chains. At initiative order 20, a paragon can use one lair action option. He or she can only use one lair action per round. As these actions greatly improve their chance of survival, most champions prefer to stay "at home".

• Instant Rest: Three times a day, a champion can use an Instant Rest as a lair action. This power grants the same effects as a long rest - but instantly.
• Champion's Call: Once a day, a paragon can send out a mental call for help in a five mile radius. The champion can transmit a message with up to five words. Any sentient creature that inhabits the champion's world-island (but no visitors from other worlds) can hear the call, understanding the message in their native language. For the next ten minutes, they will know exactly where the champion is. This does not force them to come for help, but many will.

Last but not least, the champions usually also have an arsenal of powerful equipment: Artifacts, magical or technological items they either acquired through adventuring, conquered from enemies or they received as gifts from allies and followers.

Guards and companions

In almost all cases, champions don't travel or fight alone. They are usually accompanied by powerful individuals and creatures, from high-level adventurers to dragons, celestials, fiends or aberrations. Special mounts are also common among champions.

Using the champions in your game

Except for very high-level campaigns, champions usually aren't direct enemies or companions of the player characters. Most of the time, they take on the role of plot devices - offering quests or initiating a war between worlds, but with little direct interaction between them and the adventuring group.
They are not as inaccessible as gods, and it is possible to meet them and talk to them. But such events should be the exception, and they should be moments to remember. Most of the time, adventurers only get into contact with the paragon's companions or other allies.
If the adventurers ever face a champion in battle, it should be an epic fight where the heroes have to overcome several other powerful enemies before they even get close to the paragon.