Mega Man X’s Hadouken — a fireball move borrowed directly from Street Fighter’s Ryu — is the most famous capcom easter egg in their catalogue. Obtaining it requires: reaching the fortress stages, finding the hidden capsule, and arriving at that capsule with all heart tanks, all sub-tanks full, and full health.
The Hadouken kills most bosses in one hit. It has limited uses before requiring health to be full again. It cannot be carried into stage-clears. Capcom hid the most powerful ability in the game behind a condition — full health, fully upgraded, late game — that ensured only players who had already mastered the game would receive it.
The Hadouken performs the quarter-circle forward input from Street Fighter, which is the same input used on SNES controllers for Street Fighter II released in the same period. Capcom was playing both games simultaneously and decided to make the connection physical.
The hidden capsule that delivers the Hadouken is found in Armoured Armadillo’s stage, requiring specific wall-jump positioning that the game never teaches in that area. It was discovered by players in the first months of release and has remained one of the most beloved Capcom easter eggs.
I’ve put 200 hours into this game and never caught this. The developers really reward the obsessive players.
Currently on my first run and now I have to go back and look for this. Worth the detour.