On project 1999 level is a huge factor in resisting
Note that it will never make the player completely resistant
the difference between 50 and 60 is HUGE. Even 59 and 60 is noticable.
https://wiki.project1999.com/Klandicar
is solid proof of this.
A level 59 player with 250 MR will get feared more often than a player who is level 60 with 250 MR
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On the flip side
Charm breaks feel a bit less random at 60 vs 59
Mobs resisting your spells also feels less likely at 60 vs 59 (Note)- Mobs have save vs resist too, so a mob with 500 poison resist will still resist poison spells despite your level.