Into the Unknown is an Old School game that seeks to blend the Basic & Expert rules and style of play of the '80s with the current 5th edition ruleset of the world's most popular roleplaying game.
Into the Unknown is based on 5th edition of the world's most popular roleplaying game and is fully compatible with it to the extent that you can easily create a 5th edition character and use him in a game of Into the Unknown and vice versa, without any conversion.
What sets it apart from 5th edition is that this is a "non-advanced" version that hearkens back to the B/X version of the game. It has been simplified in many places to speed up things such as character creation; "race-as-class" is back; so are old-school mechanisms such as morale, reaction rolls, 'random encounters', 'gold for XP' and henchmen.
For players used to 5e, Into the Unknown is easier and runs faster, and is an excellent game for exploring the old school style of play or introducing new players to the game.
For players used to running B/X, Into the Unknown is an opportunity to explore the latest edition of The Game without losing that Old School style of play you have been used to.