GoldenEye 007 shipped with a fully functional secret level called ‘Egyptian’ — a standalone mission set in a temple that does not appear in the film, designed entirely as a bonus for players who achieved the game’s hardest challenge.
To unlock it, you must complete every other level on 00 Agent difficulty within specific time limits — requirements so strict that several levels allow only seconds of margin for error. The Egyptian level itself is then the hardest level in the game, culminating in a confrontation with a resurrected Baron Samedi that continues even after you kill him twice.
The Egyptian level was never documented in the original manual or any Nintendo-published strategy guide. Rare left its existence entirely to player discovery. In the pre-internet era, this meant the level was essentially mythical for many players — spoken of but unverified.
The time requirements for unlocking Egyptian have been extensively documented by speedrunners since, and completing all of them legitimately remains one of the hardest non-speedrun challenges in any N64 game. Several of the required times are routinely described by experienced players as requiring near-perfect execution on difficulty settings that punish every mistake.

Saved this article for my gaming reference folder. Essential reading for anyone serious about this game.
This is exactly why I love this game. So many layers underneath the surface if you just take the time to look.