I am considering writing a "history" of the Morlocks (based on the characters in the H. G. Wells novel). Question: Is this allowed, or do I need permission from his descendants?
As of 2017, the collective works of H.G. Wells, including The Time Machine, are in the public domain. Not only that, Wikipedia also lists over a dozen stories based on The Time Machine, almost all of which were written before that date (I can only assume its US copyright expired much earlier) and only one of which is mentioned to have been authorised by Wells' estate.
Even if The Time Machine were still under copyright, you could still write that book, but you'd be limited to posting it on places like AO3 or FanFiction unless you could get permission from his estate to publish it - which is probably unlikely.
In any case, the answer is: Go for it!