Whether your game uses an Entity-Component model, Inheritence Hierarchy, or some mixin of both strategies, it doesn't dictate what a combat system is within the scope of a game.
As Qqwy points out, we're discussing the notion of some damage modifier applied to health. The damage modifier can also affect the durability of the attacked entity's gear with time too, making it less useful with every attack made. You may need to factor in resistences, temporary stat modifiers too which could make an entity more powerful or more susceptable to damage.
My advice is design your combat system very high-level like. Consider what you want your combat system to involve and how it will behave. Once you have this guideline, you'll quickly notice what makes sense to be components.
One obvious idea would be to just create a CombatStatComponent that has all your stats. This could include the base stats an avatar can have, the extra stats they can acquire possibly from gear, resistence levels the avatar presently has. The combat system uses this components data to measure the damage possible between two entities when an attack is made. When the attack occurs, an event gets sent to the damaged entity with the damage done. It simply modifies it's health component's value.
But keep in mind, all the ECS does in this scenario is describe how you organize the code that makes up this combat system. But the internal steps the system must perform to apply some damage remains the same from a generalistic point of view regardless of how you manage entities as a whole.