I beat the last level on halo 3 on legendary and nothing was unlocked?

[Robert A]

I beat the last level on halo 3 on legendary by myself and co-op and nothing was unlocked

 

[ITR Swiftness]

Did you beat the WHOLE campaign legendary or just the last level?
If just the last level try the whole game.
If you beat it legendary you should be available to see the ultimate ending.

 

[Ben]

It will only work if all missions are completed by yourself, or all missions are completed on co-op- not mixed.

 

[Leoo]

You have to do ALL levels on Legendary Difficulty or the achievement won't be unlocked.

Doing the last level won't unlock anything.

Just do CO-OP on all the maps on Legendary Difficulty.
Hope I helped.