Quote Originally Posted by AudreyChandler
Neltharion's position as "earth-warden" in the dragon pentad led to him spending a lot of time underground, keeping the rock and earth of Azeroth in order. This proved unfortunate, because the Titans had "solved" the Old God problem by shoving them into prisons buried as deep in the ground as they could manage.

Proximity to Old Gods seems to increase the magnitude of effect their powers have, so out of all the Dragons (and most other creatures on primordial Azeroth), Neltharion was exposed to the heaviest dose of raw crazy rays as he conducted his business deep underground. Not only that, but as an extremely powerful envoy/servant of the very beings that had dethroned them, he made a very tempting target to the Old Gods.

The creation of the Dragon Soul was a complicated result of him being driven insane by the Old Gods, who nudged him along on the path of how to do it.
The above is correct, also to add more to Old God involvement when Malfurion stole the Demon Soul he also heard the whispers of the Old Gods.

"Malfurion could hear the voices of the Old Gods coming from within the Demon Soul, asking him to wield the power. He used the Demon Soul to escape by opening a path through the mountain.

The more Malfurion used the Demon Soul's power, the more tainted he became with the voices. Malfurion finally broke away from the corrupting thoughts, and put the Demon Soul away, lest he be tempted to use it."