I am middle child of the Ghegorzi family. Lord of Ülyzia. A city of my father's land of Vegoznia, of the great Thaecan Empire. I am also a seasoned member of the secretive Ordo Maleficarum.

I disgrace my order by revealing its secrets, but these are dark times. The church and its sacred offices must be warned what awaits in the dark nights. To some at the church I will be branded hereticum, but I say no! You must listen.

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You could start play as a mortal and you get caught up becoming one of the monsters depending on how you play.

Types of Monsters You Could Play:

Vampire (European culture): Weak to sunlight. Weak to sunlight as an environmental effect?

Ghouls: Flesh eaters/bone eaters/eat other dead or undead. Weak to holy objects. Weapons or churches?

Liches: Weak to water. Weak to water as an environmental effect?

Werewolves: weak to silver.

Flesh Golem: Weak to fire.

Wraith: weak to magic.

Wendigo (i.e. Native American culture). Flesh eaters.

Gamplay Ideas

Dialogue choices.

When you finish speaking to an npc dont let them rehash the same dialogue. Let them say nothing after you have spoken with them. Keeps it fresh, nonrepetitive and overall more immersive.

When you open up your map let players doodle, draw on it, make it an ms paint overlay. You can draw whatever you want on it.

Ability to turn mortals into vampire spawn, ghosts, etc.

Companions that you can only get in single player. For multiplayer you already have your player friends.

Decisions matter.

Nonbuyable spells. Find them in game by doing quests.

Upgrade your health pool, magic pool, stamina pool by doing quests.

You could learn skills not depending on point spread, but on how you play the game. More Stealthy, it gets upgraded the more you use it.

You could introduce skills to make this a more rpg heavy game (skills such as):

Perception: See things you missed the higher ranks/points you have in it. Hear conversations you couldn't before or more to conversations.

Stealth

Diplomacy/Intimidation

Investigate: See and follow footprints. Outlines blood trails, etc.

Lore: Learn useful info about spells, weaknesses, and strength of enemies. Even potions that buff, debuff, or work best against certain enemies. Also a possible dialogue choice option to get in or out of situations.
 
Each sidequest should be unique and packed, dripping with flavor.

Each class of undead probably also wants to weaken the claim on the other or outright attack the other. Politics among the undead.

Entering a city as an undead automatically has the guards attack you. You have to be subtle: use stealth, mask and cloak, illusion spell, bribery, etc.
 
Healing/feeding as an undead should be difficult. Vampires have to use allure or Persuasion someone into an alley. Ghouls have to eat dead people in graveyards. Liche's have to eat souls. Wraiths have to find and eat lesser spirits. All of these risk discovery and push you into combat.