Prodoomer by PsychosilverTH is a 33 map partial conversion of Doom 2 that is known for its RPG elements and extreme level of difficulty. It was designed for the GZDoom source port, as it takes advantage of many features made possible by GZDoom such as sloped 3D floors and dynamic lights.
Prodoomer uses a hub-based layout, where the player returns to the orbital city in between each level and can travel to any level that has been unlocked. The orbital city hub allows the player to purchase ammo and weapon upgrades, recruit military units for assistance in the field, and view a bestiary of all monsters encountered so far.
The RPG system allows the player to gain experience for enemies killed. Upon each level up, new spell-like abilities will be unlocked, ranging from a simple fireball, to the ability to freeze time, to a devastating vortex that kills everything in its path. Money is gained by collecting treasure and coins, which are both dropped by enemies and found scattered throughout the levels. Bonus XP and money can be earned based on the tally of enemies, secrets, and items at the end of each level; getting 100% completion will earn the player an "S" rank in the mission, which go towards unlocking the secret levels.
In addition to the normal level secrets, there are also 60 rings hidden throughout the game. These rings are usually very well hidden and tricky to reach. Collecting all 60 is necessary to unlock the first secret level, Map 32.
Map32: Mobius (Secret Level)
Map33: Legion of Atrocity (Secret Level)