This.
An easy and effective way to introduce raid progression is to introduce a.) a new kind of damage/resistance and b.) equipment to counter that.
Example:
- The Demons of Entropy deal disintegration damage, turning their victims into dust with each hit (+50% damage of "entropy damage") to each melee/magic hit).
- Only armor hardened with diamonds can counter this damage (= new resistance which only works against entropy damage).
- Weapons with diamond edges cut cleanly through their forms (+10% damage for each weapon and armor with diamonds against Demons of Entropy).
Critigation was a bit like that, but also worked in pvp, which makes balancing more difficult and was (and still is) really hard to measure/balance in the first place.