The One Being was an ubergod that existed before the universe was created. He used the Elder Gods as slaves, so they created the Kamidogu as a weapon to kill him with.

When he died, it was sorta like the Big Bang, his body became the matter the realms are made of. But he's not completely dead, more like in a coma, sleeping. The MK universe is a dream he's having and all the people in the realms are creations of his subconscious mind.

So if all the realms were merged and the One Being were woken up, the dream would end. The realms and all life in them would cease to exist.

As for Armageddon, though, that plot wasn't actually about the One Being, he's not really mentioned in that game.

The idea behind Armageddon is that if you get too many people who use magic/superpowers together in one place, them all using those powers at the same time to fight each other will break the laws of nature and rip reality apart. In other words, it's the war in the crater that would cause the apocalypse.
(That's not a very logical story since sorcerers and Shadow Priests and the like are very common in Outworld, Edenia, and the Netherealm, surely there've been big mage-battles before and the fabric of reality didn't unravel then...but hey, whatever...)

Blaze/Taven/Daegon was supposed to kill or depower them all so the danger would be ended.