Take a Chance on Me

Chance to Cast ... / CtC

Lord of Destruction expanded 'classic' DiabloII beneath other things also with item properties which could trigger with a fairly low chance some fairly low-levelled spells, independent from actual character class wearing such an item.
That skill level is not enhanceable. The chances add, when event (the skill) and sLvl (skill's level) match. If skill is the same, but sLvl differ, the stronger sLvl seems to take precedence, if both chances are triggered at once; in such a case the spell won't trigger two times. Otherwise the chances are chained. Which triggers are 'stronger' than others is hardly explored.
Since then there are three kinds of trigger events.

1. on attack
   checks any attack (blowing)
   this mechanism doesn't work with Barbarian's Whirlwind or ranged attacks, but it works with shape-shifted druids who use ranged weapons in melee.
2. on strike
   checks any successful attack (blowing, throwing, shooting), so you must hit your opponent
   If a curse is triggered this way which lowers enemy's resistance, the curse effect won't count yet for this hit, but the next one.
   Amazon's Multiple Shot only is checked at the two middle arrows of a spread.
3. when hit
   You yourself got a thick ear, also from a spell.
   Offensive trigger spells are aimed to your nasty opponent, defensive ones onto yourself. If you suffer damage without an actual enemy causing it (traps, ground fires and suchlike) every trigger spell is aimed around you.

Most appreciated are triggers for Necromancer's curses Amplify Damage, Decrepify and Life Tap, further Sorceress' Static Field.
See german listing of items at Atair's d2wissen

Patch 1.10 added more sodding trigger business. Due to overexcited item crazyness the new events trigger stronger spells with far better chances.

4. when you level up
   checks if the unit gains a levelup
5. when you die
   checks if the unit has died
6. when you kill
   checks if the unit has killed an enemy

see also:

• Interaction of % Chance to Cast Modifiers, Basin Wiki entry by onderduiker
• how multiple CtC mods of the same skill are calculated, by tsuru at the Phrozen Keep
• % Chance to Cast tests, extended testings by onderduiker at the Basin

Don't mix up CtC effects and Charges, which cost money instead of mana and are limited, or oskills, which also are class-independent but cost mana like your own skills.
However, some triggers also cause a Cast Delay and even spoil your own hotkeys without a Cast Delay, at least graphically.
Pay attention to NextHit / NextDelay phenomen, aka NHD, too.

Some items have trigger skills with varying levels assigned.
Calculation of that sLvl follows same rules described for Charges in the separate oskills FAQtoid.

Barbarian's Whirlwind triggers none of the six mentioned chances.

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