Hades
Revelation 1:18 tells us that Jesus holds the keys of death and of hades, and orthodox interpreters regard the phrase 'death and hades' here as merely a form of hendiadys, i.e. it simply means 'death and the grave'.
I'm struggling to see this as a permissible point of view, however, because in all of the New Testament scriptures where the word 'hades' is used, it refers to a places of punishment for the soul after death (Matthew 11:23, 16:18; Luke 10:15, 16:23; Acts 2:27, 2:31).
If there are keys to what is in effect 'hell', does it not follow from this that people can be released from hell? (We would otherwise have to say that Jesus holds a key that doesn't work or which He is not able/allowed to use.) And why does Revelation 1:18 tell us that Jesus holds this key in the first place, if it will never, ever be used?
Perhaps this verse is a little stronger in terms of its support universalism than some may have originally thought.
Yes, and the problem with reading Hades as 'hell' throughout, as the AV does, is that after death and Hades have been thrown into 'the lake of fire', non-believers are also thrown into this lake of fire (Revelation 20:15), so punishment after death ('hell' by any other name) continues after Hades has been destroyed.
However, the fact that Jesus held the keys to Hades before these events must surely mean that Christ had the power to release people from Hades, something this is, of course, also made plain by 1 Peter 3:20-4:6.