Doom is like "Oh God... All hell just broke loose!" Think destruction and chaos. This is doom. If someone says they're doomed, they mean that they don't see any way to get out of the destruction and chaos that is approaching; the approaching doom. Downfall is a failure that results in a loss of position or reputation; usually from good to evil or from something good to something worse. Example: The decision to have an affair was the downfall of his marriage.

Downfall mean a failure that results in a loss of position or reputation or a A sudden decline in strength or number or importance. While doom mean a Make certain of the failure or destruction of.ior An unpleasant or disastrous destiny cowboy...

doom = prophecy of bad things downfall = when it happens

HI ^_^: downfall: the loss of a person’s money, power, social position, etc.; the thing that causes this: The sex scandal finally led to his downfall. Ç Greed was her downfall. .................... doom : death or destruction; any terrible event that you cannot avoid: to meet your doom Ç She had a sense of impending doom (= felt that sth very bad was going to happen).

Looking for help on italki is the beginning of your downfall, while listening to Eliot will be the reason for your doom. :P