Soul Progression System
This system is designed to have significant impact on gameplay over time.
It rewards dedication, constancy, specially on the long run.
The progression is split in two different systems:
Soul Cores: works like a treasure hunting, where you farm for elite 
and exalted 
non-boss creatures to obtain the monster core.
Soul Pit: makes you able to challenge the monster from the core you obtained in a arenalike pit, solo or in party.
Soul Pit Benefits
Completing a Soul Pit Challenge gives you 5% experience boost against that specific monster you just fought.
Winning the fight means that you unlocked that creature Soul Core.
By every Soul Core unlocked, you'll receive 0.1% experience boost on all soul core creatures you have unlocked.
For every 10 unlocked soul cores, you'll receive +1% additional experience boost.
It can scale up to 15% experience boost if you unlock a total of 150 soul cores, for example.
You'll also receive Soul Pit Points, which can be spent on the Soul Pit Shop.
Example: You went to hunt Dragons. You spawned a soul core portal, completed the puzzle and fought against Soul Core Dragon. You got a Dragon Soul Core from this creature (10% drop chance). Then, you sacrifice this soul core to join the Soul Pit and fight against waves of this creature. After a long battle, you complete the Soul Pit challenge. It means that you unlocked the soul core for Dragon. Now, every Dragon will give you 5.1% extra experience points. 5% for unlocking the Dragon + 0.1% for unlocking a soul core creature.
Continuing... Now, you went to hunt Demons. You were lucky enough to spawn another soul core portal. Fought against a Soul Core Demon. Got a Demon Soul Core (10% chance, same as the Dragon). You went to the Soul Pit and challenged it. You survived and complete the Soul Pit Challenge. It means that you unlocked the soul core for Demon. Now, every Demon will give you 5.2% extra experience points. And then, every Dragon will give you 5.2% extra experience points.
Why 5.2%? 5% base for completing the creature soul pit challenge. +0.1% for every soul core creature you unlocked. The Dragon you unlocked gives +0.1%, the Demon gives you more +0.1% The sum is 5% base + 0.2% from completed challenges.