Erebus burned away the bridge that connect earth and heaven and drive away 99.99% of g** that walked on earth. This evil g** goal is almost success. Humanity almost lost until the great heroes appear 

The great heroes are powerful. They can "kill" Erebus's flame to some extent but Kyle/Leo is the real counter, the only one that can kill it for good

As for you question "Erebus plan to erase the great heroes" like what I said above, the great heroes is the thorn in Erebus plan to destroy the world so the great heroes must gone especially Kyle cuz he split Erebus into 6 pieces or something 

The reason Kyle hero record is destroyed mostly is because of demon. They actively trying to find and destroy it. Another reason is because of Leo disciples, Bihar. She purposely destroy record of Heroes if it have anything related to him as shadow. (Shadow is like an assassin and do dirty work like killing traitor etc, original shadow is black dragon job) Lysinas also bound to this duty until Kyle come and took it away from her. Just like Kyle want Lysinas to stay in the light and leave all the dirty work to him, Bihar is also the same. She don't want other to know that the great heroes is a shadow.

Are you still here? The 3rd reason is because the Heroes of Dawn seal Erebus in Hero Record. They realize that they can't kill Erebus cuz they don't have Kyle/Leo power. Erebus inside the Hero Record keep burning it from the inside. Ofc the Heroes of Dawn also seal themselves inside Hero Record to hold Erebus for thousand of years. Because Hero Record original is a big book before it becomes page, meaning the record is connected with each other hence Erebus can travel to some world and burn it. The reason it can't go out cuz of Heroes of Dawn

And the reason why Leo get all the rewards is because the Great Heroes is the owner of Heroes Record

Yes most of them gone

Yes the demon did destroy most of Leo or to be more accurate Kyle (his past life) that's just how much they hate Kyle cuz he "slay" Erebus their g**. Also it's ironic that the demon race that hate Kyle/Leo the most is the one that remember Kyle/Leo while Human race don't even remember their own hero after the heroes of dawn gone they pretty much think the hero of beginning is a myth