An entry in Fortnightly Topic Challenge #39: Deep Down in the Dungeon 1.
Our heroes open the door and the remaining doors vanish accompanied by the sounds of grinding stone. The magnificent structure in the middle of the room breaks apart and piece by piece begins to line the empty corners. The blocks begin to unfold themselves in a seemingly infinite manner and the room begins to shrink. Our heroes have no choice but to enter the door they've opened reluctantly. The doorway is pitch black with nothing to be seen on the other side. As the room gets smaller and smaller, the heroes begin to rush through the door into freefall, their screams echoing through the darkness. As they embrace the ground, the faint glow of fugile worms begins to light the surrounding environment from the doorway behind them. Our heroes have fallen into a labyrinth with massive ceilings lined with the midnight glow of sapphires.
They turn towards the light and attempt to take the door out and it seals solid. Another faint glow appears on the floor bringing a new message from the tyrannical wizard:
Enter the labyrinth filled with lies, each path you see is cold and dark.
Be careful of the wandering spies, each will tear your mind apart.
They'll change the walls and the floor, no sense in marking where you've been.
Pay attention to each door, they'll let you out and keep you in.
Left is now right and down is now up, and gravity doesn't exist.
Come now my children, this one is rough, add some more souls to my list.
Realizing they have no choice but to navigate the labyrinth, the choose to press forward. After hours of wandering through the labyrinth, they approach a pair of doors. On each door there is a metal patch with engravings; the door on the left says death while the door on the right says life. After half an hour of arguing the group decides that the door on the left is the door they should choose and begin to approach it. Suddenly a voice speaks up:
I wouldn't open that one if I were you.
The group, startled, turns around to see a small featherless bird on the floor behind them.
I've lived in this labyrinth for years and everyone that opens that door is welcomed by death.
Our heroes communicate amongst themselves and determine that they should heed the bird's warning and shift towards the other door.
I wouldn't open that one either.
The bird hands the heroes a piece of parchment which reads:
The left is death, the right is life; but which is the one you believe?
The left is life, the right is death; neither of which you can see.
The left is death, the right is life; guessing will leave you all dead.
The left is life, the right is death; one of which gets you ahead.
Up is left, and right is down; left is left unseen.
Right is up, now you are stuck; to live here forever with me.
The bird then lets out an eerie laugh and flies away leaving our heroes confused.
Hints
Occasionaly while our heroes had their discussion, the bird would fly by and drop another piece of parchment containing hints; each of which added to the previous.
The left is death, the right is life; look to the left, no the right.
Whichever you choose, surely you'll lose; both of which lead you to die.
The left is life, the right is death; surely you know which to choose.
The left is death, the right is life; if right is left then you lose.
The correct door has been chosen, but the reasoning behind why that door is correct still hasn't been answered. To reach the correct conclusion pay attention to the directions given and understand that the bird isn't lying, he is simply there to confuse you.
Which door should our heroes open to advance out of the labyrinth and why should they open that door instead of the other?
Bonus Opportunity
$+50$ reputation for the first answer that successfully explains the bird's mind games.
If you like this puzzle, please go look at The Thirteen Doors of Aj Noc'la by Alconja as it was the inspiration here and this one is meant to be an unofficial continuation of it.