just a random guy that knows things

>Teal Mask

The main objective is to free Ogerpon, controlled by the teal mask. At the beginning you will have to choose one of the three heroes and defeat it in a battle to catch it.

Ogerpon - Grass/Fairy
Okidogi - Grass/Poison
Fezandipiti - Fire/Poison
Munkidori - Water/Poison

All 4 possess a special terastal "masked" form associated with a specific teratype:

Ghost for Ogerpon (final boss)
Rock for Okidogi
Psychic for Fezandipiti
Fighting for Munkidori

Gligar and Aipom get new alternative evos. Gligar evolves into a more scorpion-like creature with tiny wings, while Aipom evolves into a black monkey with a single tail and a gigantic hand on it.

We also get a regional fake inspired by the magikarp line. Basically magikarp and gyarados but lizard/dragon.

There's not really much else to say about this first half, basically it's just SV's version of the Isle of Armor.

>Indigo Disk

I know less things about the second half, but here we go:

At first, the main objective is simply to take lessons, explore all the Terarium's outposts and win the institute's tournament. There are also various side quests connected to the most important NPCs (same as with base game's professors).
After completing these tasks, you'll gain access to the true ending of SV: the Blueberry Academy is built above another underground "Area Zero", where Terapagos is located. That's all I can say about the story atm.

The "teramask" forms introduced in the Teal Mask will be extended to other mons in this half, such as Charizard (Dark) and Flygon (Bug). The teramask pokemon have a different crystal head and color palette (for example Charizard has a demonic head and is now black and white).

There aren't any new regional forms, fakes or paradox mons in this half.