Or rather, it would be more correct to say that Stanton uses the final achievement as an excuse to expose what he believes are the virtues of the title:
One of Dark Souls' defining characteristics is what is known as New Game Plus (NG+). After completion the game restarts from the beginning, except you keep almost all of your equipment and the enemies become tougher. This Groundhog Day structure is the key to Dark Souls' longevity, its narrative threads and endless depths of incidental detail. You are literally playing the same game - and yet, in practice, doing anything but. The most important thing of all has changed. You.
The game's crowning Achievement plays on this, and has the description 'The Dark Soul'. It doesn't just demonstrate the depths of Dark Souls' branching paths and many hidden secrets, but also how these odd little prizes should be used. Too many games give you achievements or trophies for simple progression. Oh, you finished the first level? Well done little man, here's a pop-up! You killed 50 enemies? Good boy! Unimaginative crap. Such games treat the idea as an afterthought, something to mark off the notches on a linear structure. But becoming the Dark Soul depends not on merely completing the game, but an increasing mastery of it.
You could choose to see it another way, as a mere completionist's prize, but it doesn't depend on acquiring everything in the game. It requires much, but all is specific. Becoming the Dark Soul will require you to (among other things) collect every rare weapon, follow every equipment upgrade path to its end, see both possible endings, and acquire all of the various magics in the game. What this boils down to, in the case of Dark Souls, is ceaselessly exploring and experimenting with each covenant and location in the game, hunting down oddities and taking liberties with other players. It does depend on doing pretty much everything - but actually figuring out what 'everything' means in this context is unique. And it'll take at least two complete playthroughs, plus a good chunk of a third.
Why? Why does Dark Souls, of all the games, pull me back after hundreds of hours in its embrace? I don't care about achievements, as a rule, but I wanted this one. I would like to tell you Dark Souls' deepest secret, but it doesn't have one. It has hundreds, and you'll never find them all. It is a tease, a puzzle to which you will never find all of the pieces, and what you discover is that it ultimately comes down to your interpretation.