The opening levels of a video game are traditionally tutorial stages, designed to ease the player into the game's atmosphere and controls by giving them only a mild challenge to overcome. With that in mind, it's not common for any major baddies to show up so early.
But it's not unheard of - and, more than that, every so often these major baddies turn out to be THE big bad themselves.
With a relatively crude understanding of the game so far, you may go into these fights and either (a) narrowly escape with your life; (b) be given your first taste of dying over and over again; or (c) get sliced up from bow to stern in a scripted sequence you have no power to stop.
These final bosses' first appearances rarely have them at full power, so you may well feel you're able to take them on.
Whatever the outcome, the fact they've deliberately shown up just to mess you around before the endgame should really give you a sense of how much of a fight you're in for further down the road.
10. Sekiro: Shadows Die Twice
Perhaps even more than its predecessors, Demon's Souls, Dark Souls and Bloodborne, Sekiro: Shadows Die Twice features an opening 'tutorial' level that tells you immediately it's not going to hold your hand.
And part of the reason for that, fittingly, is because you get it chopped off at the end of said level.
This is done by Lord Genichiro Ashina, who is thrown at you without warning as the first boss of the game. The fight is intended to be a supposed-to-lose boss battle, as there's no way you'll have got the hang of Sekiro's intricate combat system by this point - although he can be defeated if you have the patience and skill.
Whether you win or lose, though, the end of the encounter will leave Wolf one whole arm lighter.
At the end of the game, Wolf and Genichiro duel again in the same location (this is actually the third time he's fought). When bested, Genichiro kills himself, and the battle rages on with his previously deceased grandfather, Isshin Ashina, taking his place.
Some may say this doesn't count, as Isshin is technically the game's final boss. But, given that each time you die to Isshin you have to deal with Genichiro again first...yep, it totally does.