Legacy:

Legacy is a very powerful, super cool background. There isn't a logical reason not to take buckets of it.

So we're creating one.

By the book, Legacy controls how much of your past you can access mentally at any given time. This is very powerful. The background states that the mortal mind is ill equipped to hold the sheer amount of time that a demon has lived. We've elected to interpret that as saying 'the higher this background, the less other stuff you can fit in there'. Will this cause you to not be able to buy things or make them more expensive? No, but it will make things dramatically harder for you to learn past a certain point if they weren't explicitly what your Demon was made to do.

Example:
A Devourer who was built to be a horrible murderbot comes in with Legacy 5. Said Devourer remembers most of the War, names of people, events, places, etc. Said Devourer can pretty easily justify buying Brawl or physical attributes. Said Devourer will have a hard time learning other things.

Learning about new things, adapting to new situations, these are HUMAN tools, and distinctly outside the angelic build. Humans are the most malleable, adaptable things on the planet, and you sacrifice a lot of that in order to be more of what you were. This isn't a bad, it's just a unique way of playing the game.

Legacy:

The mortal mind is ill-equipped for the vast memories of the fallen, and much of what demons remember from the Age of Wrath and before is driven deep into their host's subconscious, where it can only be felt dimly, if at all. This Background allows you to recall more of your past memories than other fallen, permitting you access to more detailed information about personalities, places and events that occurred in Earth's prehistory.
Calling upon your memories requires an Intelligence + Legacy roll. The difficulty of the roll depends on the relative obscurity of the information you're after, at the discretion of the Storyteller. Remembering details about a close compatriot during the War of Wrath would require a difficulty 6, while recalling the location of a specific battlefield or the location of a former bastion might require a difficulty of 7 or higher. The number of successes determines the depth and breadth of your recollections.
0- You have only the barest recollection of your former existence.
1- You can remember some details of the war and those you fought with.
2- You can remember a fair amount of information about the Fall.
3- You remember the war and your exploits with some detail.
4- You remember a great deal about the war and the time before the Fall.
5- You have a wealth of detailed memories of your past existence.