How to get Mocha to fail a test

You can call assert.fail:

it("should return empty set of tags", function()
{
    assert.fail("actual", "expected", "Error message");
});

Also, Mocha considers the test has failed if you call the done() function with a parameter.

For example:

it("should return empty set of tags", function(done)
{
    done(new Error("Some error message here"));
});

Though the first one looks clearer to me.

In the ES2017 async/await world chai-as-promised is not needed as much. Although simple rejections are one place chai-as-promised remains a little neater to use.

A catch is required if you want to test the error in more detail.

it.only('validation should fail', async function(){
    let body = { ... }
    let rules = eventsValidation.eventCreationRules()
    let valMessages = eventsValidation.eventCreationMessages()

    try {
        await indicative.validateAll(rules, body, valMessages)
    } catch (error) {
        expect(error).to.be.instanceOf(Error)
        expect(error.message).to.match(/Oh no!/)
        return
    }
    expect.fail(null, null, 'validateAll did not reject with an error')
    // or throw new Error('validateAll did not reject with an error')
})

async/await requires Node.js 7.6+ or a compiler like Babel