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Is the thieves level progression is a bit weak in the late game or are fighters too strong?
Backstab multiplier 3-5 is pretty good at the time that you get them, but I don't think that is quite good enough on it's own. Putting it into some context, there are fighters who can hit everything for high 30s to low 40s. A top level kensai will do more than 50 damage a hit. Critical strike will double that damage, and they will do that damage 10 times per round for more than 1,000 damage. Even the average fighters like Mazzy will reach more than 100 damage per second, and Valygar can do that with a triple damage backstab. None of them will ever put as much damage as a theif into one backstab, but I think 80-100 damage 10 times per round is quite adequate when your group has something like 3 or 4 fighters capable of it. I play in SCS and often manage to kill any spellcasters before their defenses have time to activate.