Factions

Underworld Idle uses multiple Factions. Each faction creates a unique resource, and uses its own Prestige Points, which cannot be exchanged between Factions.
The lowest creature in each Faction is actually the only unit that makes the unique resource. Each unit above the first only serves to create the unit below it, and are also paid for in part by sacrificing a certain amount of the unit below it. In this way, there is functionally no difference between each tier of unit above the first.
Each unit is able to be Upgraded multiple times to double its output, and each Faction has specific Special Resources they can produce with their Faction Resources (Gems, Souls, etc.) 
== Demons ==
The first faction you will come across is the Demons Faction, which produce Gems.
=== List of Demons (Mirrors Game Order) ===
* Lilith
* Gaal
* Leraje
* Belial
* Decarabia
* Asmodeus
* Naberius
* Furfuhr
* Zaebos
* Gusion
* Crocell
* Beelzebub
* Azazel
* Belphegor
* Lolmisho
* Abaddon
* Haagenti
* Dantalion
* Agriel
=== Runes ===
These give strong early-game bonuses and cost nothing but time. These DO NOT persist on Prestige, unless the Contract "Permanent Runes" has been bought.
=== Boiler ===
This is unlocked after you've collected 75 Hell Ashes in your current run, and is used to Prestige the Demon Faction. On activation, gives Waste and Demon Faction Prestige Points. Can be upgraded with Gears.
=== Hall of Fame ===
This is unlocked by purchasing the "Demons Hall of Fame" Contract. This tab allows you to perform Challenges, which (if completed) can provide a significant boost to production of a bunch of different resources.
=== CEO Cabinet ===
This is unlocked by purchasing the "CEO Cabinet" Contract. This tab allows you to automatically purchase and assign both Fire Spheres and Fire Skulls to members of the Demon and Undead Factions. 
== Undead ==
The second faction you will come across is the Undead Faction, which produce Souls.
=== List of Undead (Mirrors Game Order) ===
* Haugbui
* Wendigo
* Khet & Sah
* Buso
* Wraith
* Jian Shi
* Utburd
* Deretnik
* Nachzehrer
* Cadaurus
* Tupilac
* Navj
* Draughr
* Revenant
* Black Widow
* Bérenice
* Cenobite
* Santa Muerte
* Pestillence
* Ghoul
=== Dark Obelisk ===
This is unlocked after you've collected 75 Bone Flour in your current run, and is used to Prestige the Undead Faction. On activation, gives Dark Mana and Undead Faction Prestige Points. Can be upgraded with Gears. 
=== Mushroom Farm ===
This is unlocked by purchasing the "Mushroom Farm" Contract. This tab allows you to purchase Mushrooms with resources (each Mushroom has different requirements) that, once grown, provide a significant amount of a particular resource. 
== Drowners ==
The third faction you will come across is the Drowner Faction, which produce Raw Mana.
TIP: These units cost Waste to make, which isn't easy to obtain very much of early-game. A general rule is to only buy as many units as you need to create the next-highest one to avoid spending unnecessary Waste.
=== List of Undead (Mirrors Game Order) ===
* Nessie
* Kikimora
* Kraken
* Siren
* Dandan
* Kelpie
* Dagon
* Mavka
* Red Herring
* Leviathan
=== Deep Oyster ===
Buyable with raw mana.
The pearl give permanent bonuses. For more details see the dedicate pages
=== The Chest ===
This is unlocked after you've collected 75 Magic Powder in your current run, and is used to Prestige the Drowner Faction. On activation, gives Energy and Drowner Faction Prestige Points. Can be upgraded with Gears.
== Warlocks ==
The fourth faction you will come across is the Warlock Faction, which produces Dark Mana. This faction is unique in the sense that it requires 20,000 (20k / 20e4) Dark Mana to unlock. 
=== List of Warlocks (Mirrors Game Order) ===
* Rasputin
* Borgia
* Hiisi
* Saltychikha
* Papa Legba
* Louhi
* Gilles de Rays
* John Dee
* Bathory
* Anansi
* Meritptah
* Locusta
* Karachun
* Imhotep
* d'Esperance
Tip: Early/Mid-game, the easiest way to create Dark Mana is to use Crystal #5, which converts all of your current Raw Mana into Dark Mana. After unlocking the Warlocks Faction and buying your first "d'Esperance" Unit, your natural Dark Mana production far outweighs the output from Crystal #5, so a good goal at this point is to be able to consistently use Crystal #5 once to afford your first Warlock, after which you shouldn't have any issue gaining Dark Mana.
=== The Grimoire ===
The Grimoire is split into different Pages, with each page having six spells, and three locked slots to put them in. You cannot purchase any spells on a page unless you have an open Spell Slot to put them in. Slots can be unlocked by spending Dark Mana, and spells can likewise be bought with Dark Mana. You can only choose three out of the six spells to activate per run, and once you finish the first page, you can access the rest of The Grimoire, with each consecutive page becoming more expensive to unlock spell slots on. 
This is probably the tab that will require the most strategy between runs, and it's recommended to experiment with different spell choices at different stages of your game, and once you hit certain milestones.