Intelligence increases damage done by weapons and catalysts through the use of a scaling mechanic. A letter denoting a weapons specific scaling properties is noted beneath the weapon's required intelligence stat. The letters range in order of effectiveness from E, D, C, B, A, and S.
Weapons and catalysts with an E rating for scaling will receive a poor benefit from investment in intelligence while weapons with an S rating receive the greatest benefit.
Note: the term Catalyst refers to any medium used to cast spells.
As is with every other stat in the game, it also has indirect bonuses such as increasing the player character's flat damage absorption. In particular, leveling Intelligence will increase the player's flat defenses against Magic damage.
Depending on the catalyst, weapon and infusion, leveling Intelligence to certain thresholds will maximize its potency with those tools.
Most weapons that inflict innate Magic damage such as the Moonlight Greatsword, Darkmoon Longbow and Aquamarine Dagger will reach peak efficiency at 40 Intelligence. As well, weapons that have been infused with a Crystal Gem or Simple Gem will also soft-cap at the same value.
For weapons that inflict innate Fire and Dark damage like the Demon's Greataxe and Onyx Blade, Intelligence will generate noticeable returns, along with Faith, up to values of 40. In the case of infusions using a Chaos Gem or Dark Gem, Intelligence soft-caps at 30, along with the same amount in Faith.
Staves benefit most from Intelligence investment. The majority of staves will not begin to gain Spell Buff until Intelligence has been leveled past 18. Once past this point though, staves will continue to increase in strength up until the 60 Intelligence mark is reached. In many cases, the rate of improvement will reduce past 45 Intelligence, so this is considered a minor soft-cap for pure Intelligence catalysts.
Dual-scaling catalysts catalysts will also benefit from leveling Intelligence, though in a different way, much like their melee analogues. Pyromancy Flames will scale with Intelligence up to 40, alongside Faith. Catalysts that specialize in casting Dark spells like Izalith Staff and Sunless Talisman increase in effectiveness up to 45 Intelligence and 45 Faith. The Crystal Chime, one of the most unique catalysts in the game, also soft-caps at 45 Intelligence and 45 Faith, though it also has a minor soft-cap at 30 Intelligence and 30 Faith.