Racing through the multiverse is one of the unique abilities of DC's most famous Scarlet Speedster, The Flash. Without Barry Allen's discovery of alternate realities, the multiverse might never have even cropped up into the canon of comic books at all -- meaning the rich, wild, varied history of the DC Universe would be so much less exciting. Not to mention that every major Crisis event includes The Flash in a pivotal role.
Recently, the rise of the super-villain Paradox has caused massive trouble in the life of Barry Allen. Traversing the timeline, and able to manipulate the multiverse itself, Paradox has been systematically removing all traces of Barry Allen from existence. As a result of Paradox's rampage through the timeline, Barry has had to come come up with some unusual methods of fighting back, including teaming up with his most prominent enemy, Eobard Thawne, the Reverse-Flash, and will most likely have to unite all of his Rogues to defeat this new threat to existence.
The opening pages of The Flash #754 reveal that in the future of Central City, Reverse-Flash has managed to finally defeat Paradox, but not without considerable destruction to the entire city. Pulling Reverse-Flash from further on in THAT reality, Barry brings him to an earlier point in the future (prior to Paradox's defeat), and together they form a plan to finally defeat Paradox by using the Reverse-Flash's Lightning Rod, which "harnesses the Speed Force and focuses it into a destructive force", and is the only weapon capable of defeating Paradox.
Paradox, meanwhile, has been traversing the multiverse, forcing Godspeed --- strapped to the Cosmic Treadmill --- to travel to different realities. There, he kills every version of Barry Allen he encounters, not only in his current reality, but in every reality in existence. From Flashpoint, through Final Crisis, we Barry is erased from existence (identifiable through his costume and those surrounding him in his final moments). By erasing Barry from existence, Paradox hopes to further fuel his power, growing into an unstoppable force that --- in this story, anyway --- won't be stopped until the 25th Century.
Creating an ultimate paradox for himself, Paradox becomes event more powerful than before, and is determined to remove not just Barry Allen, but his entire influence, and that of those who are closest to him, from reality. Paradox has already proven to be one of The Flash's most dangerous and powerful enemies, no matter how his story ends... or continues.