Chance to Hit

Every weapon attack, kick, throw and bow shot rolls to hit before it does anything else; spells do not. The roll is one formula with four numbers in it: your Attack Rating, the target's Defense, your level and the target's level. Everything that "adds attack rating" or "lowers defense" is only ever moving one of those four.

The formula

chance to hit = 100 × 2 × AR ÷ (AR + DEF) × alvl ÷ (alvl + dlvl)

AR is your attack rating after every bonus, DEF the target's defense after every reduction, alvl your level and dlvl the target's. The two halves are independent: the first is how your attack rating compares with their defense, the second how your level compares with theirs. Against a monster ten levels above you the second half alone costs about a tenth of your chance, however much attack rating you stack. The result is never below 5% and never above 95%: there is always a chance to hit and always a chance to miss.

How the two numbers are built

The Basin's order for the attacker's number: the flat attack rating (your Dexterity, the weapon's, the items', "Attack Rating (Based on Character Level)", and against demons or undead the bonuses that name them), then the percentage Bonus to Attack Rating on top. For the defender's number: the armour, the ethereal bonus, Enhanced Defense, flat defense, minus Monster Defense Per Hit, the Stone Skin bonus, Inner Sight, "% Defense", the Berserk penalty, Defense vs. Melee or Defense vs. Missile, Target Defense Reduction ("−X% Target Defense"), and last Ignore Target's Defense, which sets the whole number to 0 against the monsters it works on.

Running

The Basin's wording of the running rule: "when a player character is running, hit checks are not made for any attacks made while doing so and these attacks always hit", except for the mobile attack skills Leap, Leap Attack, Whirlwind and Charge, which roll as usual.

A level-gap you cannot close with gear: against a monster far above your level, the second half of the formula caps what any attack rating can do. Levelling is the fix, not more attack rating.

What it does not cover

Blocking is a second roll after a hit lands (Blocking). Spells and most elemental skills never roll to hit; Dealing Elemental Damage goes straight to resistance. Crushing Blow, Open Wounds and Deadly Strike only matter on an attack that has already hit (Dealing Physical Damage).