The Red Rip is an astronomical phenomenon that appears in the night sky of Scadrial. It is visible to the naked eye, and Wax describes seeing it “low on the horizon” while traveling to New Seran, but he does not specify a cardinal direction for the horizon.
The Red Rip is a possible stellar band in the galaxy within which Scadrial resides; it may appear much like how the Milky Way does in the skies of Earth, but red instead of white. That Wax barely comments on it suggests that it is a common sight in the Elendel Basin, though when it first appeared in the skies of Scadrial is uncertain. No mention of the Red Rip is made by the characters of the Era One Mistborn books, not even Sazed when talking about the religion of the Nelazan. However, since Luthadel was near Scadrial’s north pole, it is entirely possible that the Red Rip was present during the Final Empire but was far south enough in the sky that it was hidden below the horizon (the Elendel Basin is likely much further south than Luthadel was, as the poles would have returned to being ice caps once Harmony returned the planet to its proper orbit). Notably, the Red Rip is visible from other planets, known as Taln's Scar on Roshar and the Starbelt on Threnody.
During the short time that Wax was dead, (but before his soul separated from his mind and entered the Great Beyond), Harmony gave him a different perspective on Scadrial, allowing him to see it from orbit. Wax also saw “a haze of red...pressing in upon the world. He could feel it choking him, a miasma of dread and destruction.”
Wax was seeing this red haze from the Cognitive realm, without any of the restrictions or benefits that physical eyes would have. (Kelsier’s experiences in Mistborn: Secret History show us that the Physical realm looks different when viewed from the Cognitive realm.) It is entirely possible that the Red Rip and this red haze are the same thing.
VenDell comments twice that Harmony has been distracted. MeLaan also says that Harmony has indicated that a great danger is in Scadrial’s future. During Wax’s cognitive perspective of Scadrial, he noticed a shield of some sort surrounding Scadrial, protecting it from the emanations of the red haze. It seems plausible that much of Harmony’s attention is focused on maintaining the shield, as well as discerning the nature of the red haze and its threat to Scadrial.
Considering what Wax felt when looking at the red haze - “a miasma of dread and destruction” - the red haze / Red Rip could be an indication of Odium closing in on Harmony and Scadrial. Odium already has a history of killing Shard holders and splintering their Shards (Honor, Devotion, and Dominion), and Roshar has a similar astronomical phenomenon in Taln's Scar. It’s only a matter of time before he tries with Harmony.
|Era 1 books||Mistborn: The Final Empire · The Well of Ascension · The Hero of Ages · (Mistborn: Secret History · The Eleventh Metal)|
|Era 2 books||The Alloy of Law · Shadows of Self · The Bands of Mourning · The Lost Metal · (Allomancer Jak and the Pits of Eltania)|
|Shards||Preservation · Ruin · Harmony|
|History||Classical Scadrial · Final Empire · Catacendre · Elendel · Southern Scadrian|
|Magic||Metallic Arts (Allomancy, Feruchemy, Hemalurgy) · Metalborn · Mistborn · Misting · Savant · Ferring · Twinborn · Compounding · Atium · Lerasium · Ettmetal|
|Other Media||Mistborn Adventure Game · Mistborn: Birthright · Mistborn film · Mistborn: House War|