Abilities
Abilities are modularly combined from a range of triggers, targets, and effects. A trigger specifies when an ability manifests its effect, and the target specifies who is affected. There are event-based triggers, such as manual activation, and passively triggered ones such as cast an ability, hit by an attack, dodged an attack, hit an opponent, health crossed threshold, etc. These triggers trigger one-time effects, as opposed to continuous effects, which are triggered by condition-based triggers, such as less than x% health, alone, ability is active. One-time effects include things like deal X damage to target, block X damage to target, heal target by X. Continuous effects include things like increased attribute, increased HP regeneration, receive damage over time, etc. which last as long as the condition is satisfied.
An example ability would be whenever player hits an opponent, chance 25%: deal 250% damage to that opponent, which deals 350% critical hits at 25% chance; or while Health <50%, +100% Strength.
Modifiers
Modifiers make abilities more unique, and they add additional rules that change the effects of an ability. A modifier for an attribute-increasing effect could be increase effect attribute value by 10% of character's strength. An example ability with a modifier could be increase toughness by (10 + 10% strength), where increase toughness by 10 would be the base effect and the 10% strength bonus to toughness would be the modifier.
Ability cost
Abilities either cost stamina and/or mana, depending on the effects and modifiers used. Passively triggered abilities have a constant cost and they have to be enabled/disabled manually, but individual activations of their effects cost nothing. While a passive ability is disabled, it costs nothing, but can also not be triggered. Manually triggered abilities cost once per activation, and their cost is decided by the trigger's cast time. The strength of effects and modifiers, as well as the chosen triggers and targets determine an ability's cost. An active ability that has a long casting time has significantly reduced cost and can therefore support much stronger effects and modifiers. Passively triggered abilities can have their cost reduced by adding activation probabilities, so a 50% trigger chance reduces the costs by half. The event-based trigger defeats an enemy is much cheaper than hit an opponent, as it is a rarer event, and thus, would allow for more powerful effects and modifiers for the same cost.