You want to create/delete entities from a system? There is good news for you. You can just use Entities to do that. It implements SystemData so just put it in your SystemData tuple.

Don't confuse specs::Entities with specs::EntitiesRes. While the latter one is the actual resource, the other one is a type definition for Fetch<entity::Entities>.

Please note that you may never write to these Entities, so only use Fetch. Even though it's immutable, you can atomically create and delete entities with it. Just use the .create() and .delete() methods, respectively. After dynamic entity creation / deletion, a call World::maintain is necessary in order to make the changes persistent and delete associated components.

Adding or removing components can be done by modifying either components storage directly with a WriteStorage or lazily using the LazyUpdate resource.

use specs::{Component, Fetch, LazyUpdate, NullStorage, System, Entities, WriteStorage};

struct Stone;
impl Component for Stone {
    type Storage = NullStorage<Self>;
}

struct StoneCreator;
impl<'a> System<'a> for StoneCreator {
    type SystemData = (
        Entities<'a>,
        WriteStorage<'a, Stone>,
        Fetch<'a, LazyUpdate>
    );

    fn run(&mut self, (entities, stones, updater): Self::SystemData) {
        let stone = entities.create();

        // 1) Either we insert the component by writing to its storage
        stones.insert(stone, Stone);

        // 2) or we can lazily insert it with `LazyUpdate`
        updater.insert(stone, Stone);
    }
}

Note: After using LazyUpdate a call to World::maintain is necessary to actually execute the changes.