Rift the Veil

Throughout the multiverse, Shuriig and the Dark Powers working with it have empowered the Abyss and it's plague-like properties, driving the invasive plane to begin spreading across all the realms like wildfire. Its insidious influence takes the form of planar fissures, or Abyssal Rifts, which appear in the dark or forgotten corners of the wilds to fester and rot. An unaddressed rift can fast become a problem, acting as a gateway to full blown invasion from the Abyss. Much longer, and the rift will swallow the realm whole, turning it into a new lair of the seemingly infinited Abyss...

Planning your Apocalypse

The progression of a rift more or less follows the structure as laid out in Mordenkainen's Tome of Foes' section about the Blood War. If you're planning for your mission to take place during the "Initial Infection" stage of an incursion, consider running a Monster Resurgence instead to better represent the uncoordinated nature of evil's first arrival in a realm. At this point, a rift has probably not formed yet, and if the monsters can be dealt with swiftly enough, it'll remain that way.
As the incursion transitions into the "Growing Menace" stage, the rift appears and the 6 mile region around it begins to transform. For Voices looking to tell longer tales (multi-encounter adventures), this is a great way to add flavor to the adventure as you build up towards the final encounter against whatever is anchoring the rift to this realm. For the purposes of this tale, we're assuming you intend to use Shuriig's influence to color your adventure and give it detail. That said, you are welcome to replace traits and details from below with those of other demon lords (they are all actively competing with one another, as demons do, though they all ultimately help Shuriig's goal in the end) with the exception of Juiblex and Zuggtmoy. These two specific demon lords are Unavailable for Reasons. The regional effects of a demon lord can be applied around a rift no matter what your actual approval level is and whether you can use that demon lord's stat block or not.

Regional Effects

The environment around a Shuriig-borne rift changes in the following ways. If an effect would prompt creatures to make a saving throw, the DC for the save is 15 at "Growing Menace", 18 at "Stain on Reality", and 23 at "Apocalypse Now". If a creature succeeds three times (they need not be consecutive) against the same effect, they become immune for 24 hours to further saves; alternatively, three failures causes the flaw to become permanent until cured by a greater restoration spell or similar magic. A creature under the effects of a mind blank spell makes these saves with advantage; a creature under the effects of feeblemind fails automatically.

• Acidic Water. Standing water gains acidic qualities. Small animals that fall in dissolve in moments, leaving nothing behind but bones. Starting your turn in or entering such water for the first time deals 1d8+8 acid damage to you. If fully submerged, you take 4d8+32 acid damage instead.
• Sinkholes. Sinkholes appear in well-populated areas. Creatures who investigate find the sinkholes appear to be bottomless, and creatures with an Intelligence score of 3 or higher find themselves increasingly compelled to feed the sinkhole by any means, including but not limited to pushing other creatures in. A creature who sees the sinkhole rolls a Wisdom saving throw, or gain the flaw "Sacrifice. The pit calls to me. It hungers. I must feed it. Food must be fresh. Food must be living. I'll do anything to make the voices stop."
• Vanishing Population. Abandoned dwellings are common. No one comes out. No on is there. Locals might occasionally go in... but no one comes out, and no one is there. A creature that enters such a dwelling must make a Wisdom saving throw, or gain the flaw "Nihilism. There is no value in life. There is no purpose in trying. Your efforts are a waste. Nothing you do is correct. Forgiveness can only be bought with your blood. Something better will replace you."
• Filth and Spoiling. Fresh food spoils overnight. Meat rots before it's even been carved from the body. It still smells delicious. A creature who spends at least an hour within 6 miles of the rift rolls a Wisdom saving throw, or gain the flaw "Hunger. There is no amount of food that can sate me. I eat, and eat, and eat, and yet I starve. There is nothing I won't eat to try and quell the hunger."
• Crimson Rot. The land is fertile, but nothing grows. The Crimson Rot emerges from graves, chokes dark pits, and grows where it shouldn't. Locals may be seen consuming the Rot, and may also be seen with the Rot growing on and from them. Crimson rot fills the air surrounding its growths with virulent spores that latch to anything they can, seeking to devour entire settlements. When ingested or inhaled, the creature must make a Constitution saving throw or become poisoned. A creature with immunity to the poisoned condition has advantage on the saving throw, but may otherwise still be effected if the saving throw is failed. While poisoned, the creature gains the flaw "Cultivation. Rot is the great recycler. I must provide for it. It grows best on raw meat. Dead meat. Old meat." Additionally, while poisoned, the creature becomes incapable of taking a long rest as the rot prevents them from sleeping until they collapse from exhaustion (see the rules for going without sleep). A creature that does not require sleep is unaffected by this second trait. A creature that dies while poisoned this way is reanimated as an undead creature of the GM's choosing.
• Forbidden Fruit. Trees blacken and decay. They still bear fruit, though the fruit look and seem wrong to all senses. The fruit smells like honey or some other enticing aroma, but tastes both sweet and rotten. Locals are compelled to eat the fruit until none remain, which are quickly replaced. Too quickly. A creature who starts its turn within thirty feet of such a tree must make a Wisdom saving throw, or gain the flaw "Hoarding. I don't have enough food to last the season. I don't have enough food to last the night. I'll go find more food. I can't eat the food. I won't have enough to last the night."

Agents of the Devourer

Aberrations, fiends (demons), monstrosities, oozes, plants, and undead are all common servants of Shuriig, whether they're willing or not. Creatures encountered should be noted as having been overtaken or mutated by Crimson Rot. The specifics are at your discretion, but don't otherwise change the creature's mechanical traits or abilities. You're free to use other creature types to support your Tale as well; use the same stat block for the creature, but change its creature type to undead and give it the "Undead Fortitude" trait if it doesn't already have it (when reduced to 0 hit points, the creature makes a Constitution saving throw against a DC of 10 + half the damage taken. On a success, the creature is reduced to 1 hit point instead).

The Anchor

An Abyssal Rift is usually anchored to a specific creature, usually the most powerful creature present and acting as the central power or authority for the incursion. When this creature is slain, the incursion destabilizes and with it, so too does the rift. If your party is inside the rift when the anchor dies or is destroyed, they have 5 + 1d6 turns before the rift collapses and everything inside it and within 30 feet of its opening is pulled violently into the Abyss then sealed.
Sometimes, a specified object might serve as the anchor, like an amulet or a trinket. The rift remains open until the object is destroyed, which may require a casting of dispel evil and good, hallow, or some similar type of "purifying" spell. You're the GM here. Be creative.
The third type of anchor is a Dissonant Spire. Dissonant spires are living structures that grow similar to a tree and maintain the rift's stability even in the absence of other creatures. These anchors are the most dangerous, as in addition to other regional effects applied by the rift, the spire blocks all conjuration magic and any other effect that involves teleportation or planar travel (other than the rift itself). Attempts to use any of these effects results in failure, and the resource wasted. Creatures in your encounters will prioritize defending the spire above all else, even if it costs their lives. The presence of a dissonant spire is optional, but greatly enhances the difficulty of a Tale by simply being present.

Special Rewards

Treasure rewards for completing a Rift the Veil are standard, unless the mission included a Dissonant Spire. In addition the experience reward for destroying the Spire, you may include an additional Pure Chromatite for each party member in the mission's rewards.
Wraith also keeps track of how many of these missions are completed. Why? For the glory of evil, of course! No, but seriously, they are tracked.