For a moment I thought u we're looking for a random lvl up ot the type "oh! I've tripped on a root, smashed my face into a rock and now I've gained 3 levels"... scary thought if something like this really exists...
Anyway, I think you're looking for a dice roll every time you level up, like D&D rules (possible 3rd generation)... I think last game I've played something like this was TOEE (temple of elemental evil) and it was single player for PC (rules 2.5 D&D). I don't know if Neverwinter Nights has an online community (still in my "have to try" list) but why don't u take a shot at it?
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Great FREE MMORPG

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