Small edit done by me that nerfs enemy talent level scaling across the board. I take no credit for this great rework of the original Multi-Class challenge, all credit goes
    to AscendingPhoenix, find the orignal mod here: https://te4.org/games/addons/tome/dual-class-challenge

Allows the player to pick two classes, but the difficulty scales upward. The mod dramatically ramps up the difficulty level to compensate. (Original by StarKeep, forked mod by Raenarill).
You advance twice as fast, and gain six talent category points: at 10, 20, 30, 40, 50, and 60. You gain normal stat points until level 51.
BLITZ MODE: You gain two thirds more talent points. After level 66, you gain one class talent point per level and one generic every two out of three levels.
CLASSIC MODE: features base game talent points until level 100.

CLASS EVOLUTIONS: You can swap your first and second classes in the Game Options tab for evolutionary purposes.
WANDERERS AND ADVENTURERS: These are only supported as the FIRST class.
QUANTUM SLIDERS: Now supported as the second class.
EXPERIENCE ADJUSTMENT: You can now adjust how much experience you are gaining, from 50% all the way up to 500%.

TIP: Good equipment takes the usual time to show up, so you probably will see it at about lvl 50 with dual classing enabled.

In order to activate this addon, look at your Game Options for a new tab. Enjoy.

Allows the player to pick two classes, but the difficulty scales upward. (Original by StarKeep, modded by Raenarill).
You advance twice as fast, and gain two thirds more talent points. You also gain six talent category points: at 10, 20, 30, 40, 50, and 60.
After level 66, you gain one class talent point per level and one generic every two out of three levels.

In order to activate this addon, look at your Game Options for a new tab.

This mod should allow you to possess all ranks.

At the highest level of the Possess talent, it enables you to possess any rank of enemy, including Elite Bosses.

I recommend the excellent Proper Possession, as well.

More Boss!
This addon introduce 8 new bosses to the game, with their different fighting styles and special abilities.
You will see the detailed introduction about those bosses and their special abilities in the forum post.
This addon is still in Alpha test, so it is expected to have many bugs and really unbalanced settings. Hope to hear your feedbacks!

Adds 3 new dungeons to the world map (blood soaked ruins, infested ruins & wolfmire) these dungeons will have unique boss and lore spawns. 8 new bosses have been added that will spawn in the new dungeons (they will also spawn in Infinite Dungeon)

Over time I will be adding more dungeons to explore and bosses to defeat! I also have plans to create new mobs and mob variations, create unique artifacts and eventually implement a quest system to reward you for defeating the new bosses (since quests are not yet implemented bosses will drop gold upon death.

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IMPORTANT: I am unable to respond to comments on this site as my comments keep getting blocked as spam! If you have issues/bugs/requests you will need to post your comment on the STEAM Workshop addon page here: https://steamcommunity.com/sharedfiles/filedetails/?id=1769451307 So I can see/respond to you there.
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Colorizing unique+ ranked creatures on minimap

Originally separate addons by Starkeep, I take no credit for his work and spliced these two addons only to see if I can do it, and so I can tweak some numbers without my save getting invalidated. Again, all credit for the main addon body all goes to StarKeep. I merely spliced the addon together for my amusement and convenience.

Allows the player to pick multiple classes, as well as giving the option to spawn rares and randbosses, and control loot quality and spawn rate. Added a setting for the experience penalty from MultiClass Challenge.

In order to activate this addon, go to your Game Options for a new tab.

Allows the player to pick multiple classes, turning all enemies into Randbosses as a counter-measure.

In order to activate this addon, look at your Game Options for a new tab.
EXP penalty you suffer from using this addon has been modified, from 0.5 to 0.85 (You only lose 15% EXP now)
Original Author is StarKeep, all credits go to him, I do not take credit for any of his work, I only modified the EXP penalty. - Rae
Updated Version 1.4.1:
- Possessor and Adventurer can now be chosen in the Multiclass menu

Allows the player to pick multiple classes, turning all enemies into Randbosses as a counter-measure.

In order to activate this addon, check your Game Options for a new tab.