Cosmos
The Cosmos is the void between the Ten Realms of Kaldheim. It is a nebulous space filled with shifting lights like an aurora. Alrund is the god of the Cosmos.
The World Tree
The World Tree exists in the Cosmos. It is the origin of all life on Kaldheim, and you can see the branches in the sky in all ten realms.
Cosmos Monsters
Magical creatures known as Cosmos monsters live and thrive in the space between the realms. Most of them resemble a recognizable animal, such as a wolf or eagle, but grown to a prodigious size and enhanced with mystical abilities and supernatural knowledge. These monsters are incredibly old. They are said to be the first "fruits" of the World Tree, and they now serve as its guardians, maintaining the structure of the plane and its realms. Most of the Cosmos monsters can move freely between the realms, often bringing havoc and destruction with them when they enter a realm.
The Cosmos monsters share little in common besides dwelling in the Cosmos. Their numbers include wolves, ravens, bears, horses, and a single squirrel, and they vary in size from the unimaginably massive Cosmos Serpent to the tiny, mischievous Toski. Many of the Cosmos monsters like Sarulf are well-known, appearing in numerous sagas, while others have never been seen and their deeds are unknown.
The peoples of Kaldheim believe that when the Cosmos monsters were born, all the secrets of the world — including events that hadn't yet happened — were locked inside their minds. Mages from many realms, and even gods, have devoted their lives to unlocking those secrets. By making every Cosmos monster submit to him one by one over centuries, the god Alrund was able to take their secrets from them.
Notable Cosmos Monsters
Omenpaths
It was challenging, but not impossible, for denizens of one realm to cross the Cosmos and enter another realm. As realms shifted on the World Tree, passages called Omenpaths sometimes opened between realms. Inhabitants of one realm could use an Omenpath to reach another one. Sometimes this was as simple as walking through a shimmering light on the landscape, but other Omenpaths were more treacherous and involved crawling through shifting tunnels and being deposited unexpectedly on the mountaintop of an unfamiliar realm. Once an Omenpath between realms had opened, some powerful mages could close and reopen them at will. The opening of an Omenpath often was a precursor to the two realms overlapping in a violent Doomskar.
Because of the Cosmos Elixir that makes the gods what they are, the gods of Kaldheim were immune to the disorienting effects of the Cosmos and could cross between the realms more easily than other races. Shapeshifters' ability to change form and the Omenseekers' rune-enhanced longboats also enabled passage between realms. On the other hand, a Planeswalker's ability to travel to other planes could not be used to traverse the Cosmos, with Kaya requiring Cosima's longboat and Tibalt using the Sword of the Realms.
During New Phyrexia's Invasion of the Multiverse Kaldheim's inhabitants burned the World Tree to deny Phyrexia access to it. However, Realmbreaker created a new sort of Omenpaths that became a Multiversal phenomenon.