Complex sentence: They were believed extinct after the Asgardians stopped an attempt to do that five thousand years ago, but unbeknownst to the Asgardians, Malekith and a number of warriors escaped and went into stasis until another attempt would be possible.
Split into simpler sentences: They were believed extinct after the Asgardians stopped an attempt to do that five thousand years ago. Unbeknownst to the Asgardians, Malekith and a number of warriors escaped and went into stasis until another attempt would be possible.