If you wanted to make the sentence plural, you could say, "some terrible things have happened," or "no terrible things have happened." It would depend on the content of the conversation.
Something terrible has happened, hasn't it ? - the first one is correct.
I agree with missyasmine, the use of "they" in the second sentence is not proper unless in the first part you have mentioned someone or a bunch of people. For example:
The prisoners have escaped last night, haven't they?